MSF5000 TX Idle?

[bellersley]

Hi group. I'm working on an MSF5000 and I've got a question about the TX Idle frequency. I notice that when the radio isn't transmitting anything, there is a very low amount of RF coming from inside the cabinet on whatever the TX Idle frequency is programmed to. While I understand this is normal, I'm curious as to why it's there. I've heard opinions that it's there to keep the VCO "happy" and ready to go, but that doesn't seem to make sense considering it's all solid state.

[wavetar]

According to the help files in the MTR2000 (will be the same for MSF5000):

This frequency is used in simplex operation to shift the transmitter VCO to another frequency to avoid distortion during receive mode.

Recommended value for the TX idle frequency = TX frequency-50KHz

If the TX & RX frequencies are different, the TX idle should be set to equal the TX frequency.

Todd

[bernie]

My two bits worth:
To answer your question, the VCO does not instantly produce the required output when powered up.

It takes a certain amount of time for the VCO to settle in on frequency.
The phase lock loop has a "loop filter" to prevent oscillation in the control loop.

Much faster to just move it 50KC or so to keep it from interfering with an on channel receiver, such as would be found on a simplex base, or two receiver set up.

[Nand]

I've heard opinions that it's there to keep the VCO "happy" and ready to go, but that doesn't seem to make sense considering it's all solid state.

Well, that is one way to put it. But in reality, the VCOs are running all the time. This is why you can hear it and also see it when you look at the two green LEDs indicating that both VCOs are running and locked on frequency. I also prefer to have the idle frequency some 50 kHz away from the desired TX frequency because I don’t like to hear it on the portable when the repeater isn’t actually keyed up. The time it takes to get on the actual TX frequency is not noticeable since other (programmable) delays also come into play.

Nand.

[bellersley]

So I take it the very slight output of the VCO isn't even finding it's way to the antenna, so I don't need to be concerned with it radiating a low level signal, aside from inside the unit itself?

[Nand]

So I take it the very slight output of the VCO isn't even finding it's way to the antenna, so I don't need to be concerned with it radiating a low level signal, aside from inside the unit itself?

That is right. None of the RF amplifier stages are powered so no RF is getting out the antenna.

You also should be able to pick up some signal from the receive VCO. It is running at a frequency 10.7 MHz lower than the RX frequency.

Nand.

[bellersley]

Excellent, thanks all for the explanation. Repeaters are a relativly new avenue for me, and I've been tasked with getting this thing on the air. I was worried something had gone screwy but I'm glad to hear that's not the case!

Now if I can figure out how to make this require an MDC RAC, I'd be all set! (no, it doesn't have that option board unfortunatly)