External views of the Aural Carrier Transmitter
"It is our belief that the aural subcarrier
no business being modulated in the video exciter... at least on a
Adding the aural subcarrier to the baseband video causes several
Any nonlinearities result in additional intermodulation products,
in the infamous 'triple-beat.' If triple-beat isn't enough, then sync
can result in vertical sync buzz on the audio. Perhaps the worst effect
of modulating the audio with the video at a repeater is the myriad of
products scattered all around the spectrum, modulated with the audio
probably getting into every nearby receiver. A plus of having a
transmitter is that there isn't the VSB image audio carrier to contend
This transmitter is capable of over 25 watts output power
to allow one to vary the aural/visual power carrier ratios. In our
the aural carrier will be combined with the visual carrier with a
coupler (See the Xmit
Filters and Combiners page for more info.) This coupler offers less
than 1db of insertion loss to the visual carrier but about 9db to the
carrier. Keep in mind that "FM Power" is cheap compared to linear AM
so enough power is pushed to provide the proper power ratios.
This transmitter generates its carrier on-frequency using an MCL POS-535 VCO module. The output of the VCO is amplified by a MAV-11 and then by an MRF-559 to several hundred milliwatts and applied to a Mitsubishi M57729 power module.
A dual-modulus divide-by-N counter is used to divide the carrier
down to the 62.5 KHz reference frequency. This is compared to the
derived from a 5 MHz crystal oscillator.
The output of the phase/frequency comparator is applied to a loop filter/integrator. The audio is bandpass filtered for a 60 Hz to 13 KHz bandpass and combined with the output of the integrator. It is important that the lowest modulated frequency is above the corner frequency of the PLL's loop filter: Failure to do this will result in modulation causing loop instabilities. Because the VCO is only being frequency-controlled (that is, the loop filter wasn't being used to eliminate phase noise, etc.) extraordinary measures had to be taken to keep extraneous noise from the VCO: The VCO has it's own, private voltage regulator. It is also mounted in its own die-cast enclose which is insulated mechanically from the main enclosure with foam, to prevent microphonics. A note: The noise produced by the VCO's own 7812 regulator resulted in very audible noise on the received signal, so additional active (e.g. resistor, capacitor, and pass transistor) as well as passive (large-value capacitor) filtering needed to be added to eliminate this problem.
The controller provides an interlock such that if the transmitter's frequency control loop is unlocked the transmitter will not key (or will unkey if it unlocks during transmission.) Because of the long time-constant of the loop filter/VCO chain, it can take as long as 15 seconds for the PLL to lock up. To key the transmitter during this lock time could cause a signal to appear who-knows-where... Because of the long initial lock time, power is applied continuously so that the synthesizer remains locked at all times, ready to be keyed on a moment's notice. This circuit was complicated a bit by the fact that when the amplifier is enabled, the PLL goes out of lock momentarily (due to 8-12 amps suddenly flowing around the enclosure causing a 200-500 Hz shift in frequency.) Additional logic had to be added so that the "unlock" condition would be ignored for a short time after key-up, or else the transmitter would continuously key, unlock, unkey, then relock, key, unlock, unkey...
If I were to build this transmitter again, there are a few things that I would do differently:
Keep watching this page, as it will be updated as time goes on...
Do you have any questions/comments about what you have just read? If so, please email me and make an ask of yourself...
Return to the New WB7FID ATV Repeater Page...