In this section, I am going to talk about the communications subsystem of the LM as it is described in the LM's handbook. I am going to show all the absurdities and incoherences it contains. But first I must teach some notions of electronics which are necessary to understand why this subsystem is absurd. If these notions are familiar to you, you may skip them.

I'll first talk about the way received signals are separated. Low frequency signals can't propagate all alone though space; their low frequency makes them unfit for that purpose. On the other side, high frequency signals are able to propagate, but their frequency is too high to be audible. So how is this dilemma solved?

This problem is solved by modulating a high frequency signal, called the carrier, with the low frequency signal which contains the relevant information (voice or music for instance). The carrier has a high enough frequency to propagate through space. There are several type of modulations: 1) the amplitude modulation consists in changing the amplitude of the high frequency carrier according to the low frequency signal. This was the first type of modulation used. However, this type of modulation can be a little affected by noise.

2) The frequency modulation consists in slightly changing the frequency of the carrier according to the low frequency signal. In this type of the modulation, only the frequency (slightly) changes, but not its amplitude which remains constant. This type of modulation is more immune to noise than the previous type, and gives a sound of excellent quality. However the range of propagation is diminished comparatively with the previous type of modulation, and needs more relays.

The phase modulation consists in modulating the phase of the carrier according to the low frequency signal. Like in the frequency modulation, the amplitude of the carrier remains constant. The phase represents the time that a period of the wave starts. For instance, the two signals I represent here are in phase. And the two signals I represent here are dephased of 180°

But a receiving antenna can receive several signals emitted from different sources, and contain different informations which must be separated and received independently. So, how is this separation made?

When two independent sheathed wires are are wound around a common core, if a variable current is applied on one of the wires, then this variable current is also found on the other wire, although there is no direct electrical contact between them. That's called magnetic induction.

So, if we send our high frequency signals on one of the coils, we'll also find these signals on the other coil. But you'll say that we still haven't solved our problem of signal separation, for the two signals are still mixed on the other coil.

Yes, but if we apply a capacitor on the two ends of the other coil, then the signals are stifled on the other coil. Well, we now have stifled both signals, it still doesn't solve our problem of signal separation.

But in fact, it does, for if we apply a capacitor on the ends of a coil, that we load the capacitor with a battery, and then make it discharge through the coil, there will be the creation of an alternating current of a given frequency which depends both oh the inductance of the coil, and the capacity of the capacitor; this frequency is called resonance frequency of the LC circuit.

If if is not easy to make the inductance adjustable, it's different with the capacity of the capacitor. It's possible to make the capacity of a capacitor vary with two sets of metallic plates which pivot relatively to each other.

If we apply a high frequency signal of the first coil and a variable capacitor is applied on the ends of the second coil, the signal which appears on the second coil will be stifled, except when the capacitor is adjusted so that the resonance frequency of the LC circuit matches the frequency of the signal which is applied on the first coil.

Given this property, it then becomes obvious how the variable capacitor allows to separate the two incoming signals: When it is the first signal which is to be selected, the capacitor is adjusted so that the resonance of the LC circuit matches the frequency of the first signal, and conversely, when it is the second signal which is to be selected, the capacitor is adjusted so that the resonance of the LC circuit matches the frequency of the second signal. You'll say that the carrier which is obtained on the second coil is still a HF signal which is unfit for being listened to, but this carrier can easily be eliminated by a low-pass filter which lets only pass the low frequency signal which may then be listened to, after appropriate amplification. You may point out that one of the modes of modulation I described uses frequency modulation, and wonder if, when the frequency varies, the signal is not stifled by the RC circuit along its variation? No, in fact, for the variation of the frequency is slight and allows the signal to be accepted and not stifled by the LC circuit on its whole range of variation. The radioreceiver that I show here is of course much simplified, and just purposed to give an idea of the way it works. This mode of signal selection of course supposes that the two signals use carriers of different frequencies. The separation of the signals would not be possible if the two carriers were using the same frequency. That's why it is absolutely mandatory that the radio stations all emit on different frequencies. Two radio stations which would use frequencies which are identical or too close would not be clearly heard, and each one would be a cacophony to hear.

What is also frequently used in electronics is a crystal oscillator which allows to obtain a very stable frequency which does not vary. An electronic circuit allows to produce a variable current of a given frequency which depends on the adjustment of this circuit. When a special device, called "crystal oscillator" is inserted in this circuit, it forces the oscillator to have a very definite frequency, which is extremely stable and shows no (or a negligible) variation.

I must also have some words on the radar. A radar sends a wave to space; when the wave bumps on an object, the wave comes back to the radar; the radar then knows it is pointing at that object, and by measuring the time between the emission of the signal and its reception, it can know at what distance the object is: Indeed, knowing the speed of propagation of the signal (the light speed), it is possible to convert the travel time of the signal into a distance.

Generally the signal of the radar just bumps on the object it tracks. But in some case, this object may be an intelligent object (such as the LM) which is equipped with a transponder. A transponder is a device which retransmits the signal it receives. The advantage of this is that the radar will receive a signal which is cleaner and stronger than if the signal was just bumping on the object.

Now, if the signal sent by the radar was continuous, it would not possible for the radar to know when started the signal it receives, and so it would not be able to determine the travel time of the signal. That's why, instead of sending a continuous signal, the radar sends instead bursts of the signal separated by silences. That way, when a burst comes back, knowing the time that the burst started, the radar is able to measure the time between the emission of the burst and its reception.

Eventually, the transmitted signal may be continuous when it is retransmitted by a transponder, but in this case it contains information which gives timing indications; here I show this signal modulating the carrier in amplitude because it is easier to represent, but it more generally modulates the carrier in frequency (or phase).

Now, let's start with the analysis of the communication subsystem of the LM as described in the LM handbook.

They say that, in what concerns the communications between the LM and the CM, if the LM is seen from earth, channel B is used for carrying voice backup, and in the opposite case for carrying telemetry signals. What has it got to do with the fact that the LM is seen from earth, since it concerns the communication of the LM with the CM and not with earth?

On this schema, we can see that there are signals which use a carrier of the same frequency (for example, the LM transmits voice backup and data on carriers of the same frequency -259.7 mc-, and the earth transmits voice/backup and voice/prn on carriers of the same frequency -2101.8 mc).

I have already explained that the carriers must use different frequencies, so that the signals which modulate them can be separated.

They say that the PCM data can be transmitted to earth on a high bit rate (51.2kb/sec) or a low bit rate (1.6kb/sec). The PCM data can also be transmitted to the CSM via the 259.7mc carrier but only at the low bit rate! Why, since this frequency would also allow to carry the high bit rate?

They say that the transponder receives on a frequency of 2101.8 mc and retransmits on a frequency of 2282.6 mc. But in fact a transponder retransmits a signal on the same frequency it receives it; it doesn't change its frequency.

In this schema extracted for the Lem's handbook, Astronaut A cannot transmit data either to the other astronaut, nor to the rover, whereas astronaut B can transmit data both to astronaut A and the rover.

Astronaut A should be able at least to transmit data either to the other astronaut (who could retransmit it to the rover)... ...or directly to the rover. More logically he should be able, like astronaut B, to transmit data both to the rover and the other astronaut.

On the same schema, we can see that the rover can transmit data to the earth, but cannot receive any from earth, whereas the LM can both transmit data to and receive data from earth. The rover cannot transmit data directly to the LM which is just nearby... ...and cannot either receive any from the LM (which would indirectly allow it to receive data from earth through the LM which can). There is no reason why the rover should not be able to send data to and receive data from the LM!

They first say: "Range is determined by the time it takes to the signal to travel from the MSFN to the LM and back to the MSFN". This is correct, but later they say: "Once the range has been established accurately by the PRN code, the code is discontinued and range is updated continuously by the Doppler technique; the comparison of phase of the received signal with the phase of a local oscillator allows to adjust the frequency of the local oscillator to put it in phase with the received signal". The doppler effect is what you can experiment when a horning car is passing along: As it moves away from you, the tone of the horn changes; this is due to the change of shape of the sound wave. This effect is called doppler effect, and is used by the police to measure the speed of cars. Theoretically it would be possible to update the range from the last acquired range obtained with the PRN and integrating the speed of the target obtained with the doppler effect. If could work if this speed was absolutely precise; but it is not the case; The speed obtained from the doppler is an approached value, and there is a little error on it; on the range obtained with the PRN, there is also a little error, but the difference is that, unlike for the PRN, the error on the doppler is integrated and thus diverges; it rapidly becomes very important, at such point that, after a moment, it becomes unacceptable; the radar then becomes totally inoperational. That's why the PRN code must absolutely not be discontinued for the radar to continue to work correctly and be usable.

The frequency of the VCO of the FM modulator is stabilized with a PLL in order to eliminate the noise in the LF signal. Here we have an example of a normal frequency modulator. The PLL looks like the one of the frequency modulator of Apollo, but with an important difference though: the frequencies of the VCO and the crystal oscillator are multiplied instead of being divided before being inputted into the differentiator of the PLL. On the frequency modulator of Apollo, we can see that the signal of the VCO is divided twice on the right branch of the PLL, a first time by 4, and a second time by 2048, which gives a total division of 8192; the resulting signal has a frequency of 76MHz/8192=9.277KHz. On the left branch of the PLL, the signal of the crystal oscillator is divided by 512, which gives a frequency of 4.7MHz/512=9.17KHz; it is close and a little less than the signal coming from the VCO. So, what is the problem? The problem is that what is stabilized is not the original frequency of the VCO, but its frequency divided by 8192; so you may ask: If its frequency divided by 8192 is stabilized, is its frequency not also stabilized? This example shows that a signal obtained by frequency division of a signal can be stable without the latter signal being stable itself. The upper part shows a perfectly stable signal (colored in green); the signal obtained by a frequency division of 16 (colored in blue) is naturally also stable. The lower part shows a signal which is less stable (colored in red), with variations of frequency, and yet the signal obtained by a frequency division of 16 (colored in orange) is as stable as the divided signal of the upper part; this comes from the fact that, within a period of the divided signal, the deceleration of the signal is compensated by an equivalent acceleration of this signal, so that there are the same number of periods of this signal within a period of the divided signal. That means that, the more the signal of the VCO is divided in the PLL (and the signal of the crystal oscillator correspondingly divided to obtain a match of frequencies), and the less well the carrier is stabilized. In fact the PLL can only regulate frequencies which are under the matching frequency of the PLL (the signal which comes from the crystal oscillator), not those which are above it; that's why the matching frequency must always be way above the maximum frequency of the LF modulating signal. In this FM modulator, the matching frequency is five times the frequency of the VCO. With this matching frequency, the PLL completely eliminates the noise on the modulating signal, even if it has a large bandwidth. The frequency division of 8192 of the VCO signal in the PLL of the FM modulator of Apollo is more than excessive; it generates a important noise in the modulating signal, for the matching frequency of the PLL, 9.3KHz, is inside the bandwith of the modulating signal and not above it. Indeed, if voice, which is in an accepted bandwith of 300Hz to 3.4Khz (some talented singers can go higher, but I don't think the astronauts has very high-pitched voices), can avoid the polluting noise, such is not the case of the biomed data and the video signals; the biomed data is announced with a frequency of 14.5 Khz, and the video data can go up to 500Khz; the noise on the VCO is going to seriously pollute them, because the matching frequency of the PLL is too low to eliminate this noise. If the signal from the crystal oscillator had not been divided, and the signal coming from he VCO had been divided 512 times less, that is only by 16, the matching frequency would have been 4.7MHz, high enough to eliminate the noise on the whole bandwith of the modulating signal (the matching frequency of the PLL would still have been 10 times higher than the maximum frequency of the video signal). It could even have been better: The signal of the VCO could have been not divided, and the frequency of the crystal oscillator could have been multiplied by 16; the matching frequency would have been close to the frequency of the VCO, totally eliminating the noise on it, even in high frequencies. That means that this exaggeration in the division of frequencies creates an important noise on the carrier which makes that the FM modulator is going to work awfully bad (especially on the biomed data and the video signals).

Then they say that the output of the VCO which is frequency modulated is multiplied in frequency, and phase modulated this time by the PRN code. A carrier cannot be modulated both in frequency and in phase; the resulting signal would be incoherent.

Then they say that this resulting signal is multiplied to the high frequency of the carrier. This is absurd; this is directly the high frequency carrier which should have been modulated by the signal it is supposed to convey, either in frequency, or in phase, but not both. The fact of multiplying the frequency of the already modulated carrier, also multiplies the frequency of the modulating signal. (In my animation, I show an amplitude modulated signal because it is more demonstrative).

They say this: "In the emergency-keying mode, only one astronaut may key at a time". They could key at the same time, if each astronaut had his own channel for the emergency-keying mode. Now, if the Emergency-keying mode uses a common line, that means that it is not possible to know which astronaut used it.

They say this: "The S-band receivers receive a 2101.8 mc carrier which is phase modulated by the 30 and 70kc subcarriers and the PRN ranging code". if all those informations are carried by the same carrier, it will not be possible to separate them.

They say that the Audio input is clipped... But it cannot be clipped, for it would lose information. If it was a binary input, or a signal modulated in frequency by the voice, clipping would be possible, but not in this case in which the audio modulates the signal in amplitude.

They say: "The 30kc oscillator (circled in red) in the modulator continuously modulates the transmitter. This continuous modulation sustains the receiver AGC level, causing it to remain quiet during speech-pause periods". But the output of this oscillator is continuously mixed either with the voice, or with data (and there are no "speech-pause periods" in data). In fact this oscillator is useless and pollutes the signal that is mixed with.

They say that the output of a crystal oscillator is mixed with the received VHF signal. It is in fact completely useless, for mixing signals is made within a transmitter (in order to modulate a HF carrier with a LF signal to carry for instance), but not within a receiver.

They say: "The PMP also permits the LM to be used as a relay station between the CSM and MSFN, and, for EVA programmed missions, between the EVA and MSFN". This is a lie; for I have previously shown that there was no communication link between the LM and the Rover.

They say: "This 512kc square wave is routed to a frequency doubler whose output is a 1.024mc sine wave". A frequency doubler simply doubles the frequency of a signal; so the 1.024mc output should still be a square wave, and not a sine wave.

They say: "This 512kc signal is added to a 113-kc signal and the result is doubled to obtain the 1.25mc (1250kc) subcarrier". Why not simply tune the oscillator directly on 1250kc instead of 113 kc to obtain the subcarrier? It would save a balanced mixer and a frequency doubler!

They say: "The EKG data frequency modulates a 14.5kc subcarrier oscillator (circled in red). The oscillator output is mixed with voice (which comes out of the low pass filter circled in orange); the composite signal modulates the 1.25mc subcarrier in the same manner as voice for the S-band transmission." If the frequency modulated data is mixed with voice before modulating a subcarrier, it will not be possible to separate it from voice at the reception of the signal.

They say: "The VHF channel B input has high and low pass filters (circled in red) to separate voice and extravehicular mobility data". I don't see how these filters allow to separate these signals, for voice covers a wide range of frequencies.

They say: "the PM mixing network can process a composite of voice, biomed data, PCM data and EMU, all on the same 1.25mc subcarrier." If all these signals use the same subcarrier, it will not be possible to separate them.

They say: "If one audio center fails, the astronaut affected by the malfunction can switch to the other audio center." I wonder how the astronaut can switch to the other audio center? How will he share it with the other astronaut?

They say: "When the DUA is in the DATA mode, an audio inhibit circuit (circled in red) supplies an inhibit signal to the voice amplifier. This prevents extraneous noise that may happen on the uplink carrier from entering the audio center (through the Audio amplifier circled in orange)." Oh, I love the explanation, so humorous! In fact, the audio amplifier is inhibited by the audio inhibit circuit for the simple reason that, in the data mode, it is the data it amplifies, and not the voice.

they say that the 3.95kc tone signal initially received is retransmitted (and blocked by the 5.267kc band pass filter). Then the CSM sends a 5.267kc signal which passes through the 5.267kc band pass filter (circled in green). It goes to a product detector (circled in orange) of which the output controls the frequency of a local high frequency oscillator (circled in red). The frequency of this oscillator is divided down to 252 and 126kc (circuit circled in orange). The 126kc signal is divided by 4 (31.6kc down counter - circled in red) to produce a 31.6kc signal which goes to the early gate (circled in dark blue). The 31.6kc signal is delayed by 4ms (by the circuit circled in green), and the delayed signal goes to the late gate (circled in light blue) and also to a divider (circled in violet) which divides the frequency by 6 to provide the 5.267kc down counter signal; two outputs of this down counter signal are used to alternately allow or inhibit the early and late gates. The outputs of the early and late gates are mixed to produce a composite signal (circled in red) which is used to fabricate the reception ranging data. The delayed 31.6kc signal (circled in blue) is used to fabricate the transmission ranging data. The composite signal fabricated from the local oscillator is mixed with the 5.267 kc incoming signal (sent by the CM) in order to obtain the reception ranging data.

Absurdities: 1) It's up to the transmitter to provide the ranging data and not to the receiver! 2) The width of a cycle of the signal which stands for the ranging data (16 microseconds) is shorter than the period of the received signal (192 microseconds) when it should be the converse. 3) The product detector (circled in red) receives the 5.267kc delay reference signal (from the circuit circled in orange), which is the 28 microseconds delayed signal from the down counter signal (which is itself 4 microseconds delayed from the 31.6 kc signal, which makes that the delay reference signal is 32 microseconds delayed from the 31.6kc signal, which represents a full period of this signal; in other words, the delay reference signal could have been obtained by dividing by 6 the 31.6kc signal). It is produced from the local oscillator. The other input to the product detector is normally the 5.267kc received signal which goes through a 5.267 kc band-pass filter (circled in blue); but the fact that this signal is cut by a signal of higher frequency (the composite signal created from the early and late gates we saw above) makes that its frequency is altered, and is in fact blocked by the 5.267 kc band-pass filter. In other words, the local oscillator controls itself, and produces a signal which is independent from the (5.267 kc) signal sent by the CSM. 4) The retransmitted ranging data is produced locally and has nothing to do with the signal sent by the CSM, is completely uncorrelated from it. This means that the CSM cannot use it for the ranging.