The new WB7FID repeater:

The 70 cm Aural Carrier Transmitter

External view of Aural TransmitterExternal view of ATV aural transmitter - side view
External views of the Aural Carrier Transmitter

"It is our belief that the aural subcarrier has no business being modulated in the video exciter... at least on a repeater..."
Aural transmitter controller board
Aural Carrier transmitter controller/reference board 

Adding the aural subcarrier to the baseband video causes several problems: Any nonlinearities result in additional intermodulation products, possibly in the infamous 'triple-beat.' If triple-beat isn't enough, then sync compression 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 low-level products scattered all around the spectrum, modulated with the audio program, probably getting into every nearby receiver. A plus of having a separate transmitter is that there isn't the VSB image audio carrier to contend with...
Aural transmitter- internal view of VCO assembly
Aural Carrier transmitter VCO assembly

This transmitter is capable of over 25 watts output power (adjustable) to allow one to vary the aural/visual power carrier ratios. In our installation, the aural carrier will be combined with the visual carrier with a directional 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 Aural carrier. Keep in mind that "FM Power" is cheap compared to linear AM power so enough power is pushed to provide the proper power ratios.
Internal view of Aural Carrier Transmitter
Internal view of aural transmitter

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 frequency down to the 62.5 KHz reference frequency. This is compared to the reference derived from a 5 MHz crystal oscillator.
View of power amp assembly
The power amplifier module assembly

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...

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Updated 991213