VHF Transmitter


#1

The project I’m working on (an SDR for a spacecraft
communication groundstation) needs to transmit VHF at
around 145 MHz. There seems to be a distinct “gap”
in the transmit capabilities of the USRP daughterboards
between 32 MHz and 400 MHz.

I’m looking for suggestions as to how to best accomplish
this. My initial thought was to output at 10 MHz using
an LFTX board, and upconvert to 145 MHz with an external
mixer, LO, and filter. This seems straightforward, but
would be a custom implementation, and my project wants
to stay with commercial-off-the-shelf if possible.

Another suggestion was to skip the modulation (GMSK) and
output the baseband signal with an LFTX. This could then
be fed into a commercial off-the-shelf VHF FM transmitter.

I was also wondering if it would be possible to modify
an RFX-400 daughterboard to go down that low (Matt?).

Anyway, I was hoping to hear what people thought of these
suggestions, and any others they might have.

@(^.^)@ Ed


#2

What about BasicTX, which does 1-250MHz?

Dimitris S.
“If you think you’re too small to make a difference, try sleeping with
a mosquito!” - Amnesty International


#3

BasicTX allow transmit only up to 44MHz, isn’t it ?

Pavol Duris


#4

Dimitris S. wrote:

What about BasicTX, which does 1-250MHz?

The 250 MHz spec on the BasicTX is somewhat misleading.
Although the analog portion (basically the transformer)
is rated to that, the D/A converter is only running at
128 Megasamples/sec. Nyquist says I can only use that
to synthesize up to a 64 MHz signal. Beyond that, I would
have to under-sample and use one of the aliases. Essentially,
to get 145 MHz, I would have to run at a lower frequency
and try to use one of the upper aliases and a high-pass
filter to get 145 MHz.

Is this practical? Is there enough energy there to have
a decent S/N ratio after filtering?

@(^.^)@ Ed


#5

Engin K. wrote:

WBX0510 board will be released soon, you can wait it.

Last I heard, the WBX0510 won’t be released until
sometime in 2009. The spacecraft is being launched
this Feb, so we need the groundstation up and running
before then. We really need a solution I can go with
right now.

@(^.^)@ Ed


#6

Ed Criscuolo wrote:

have to under-sample and use one of the aliases. Essentially,
to get 145 MHz, I would have to run at a lower frequency
and try to use one of the upper aliases and a high-pass
filter to get 145 MHz.

Is this practical? Is there enough energy there to have
a decent S/N ratio after filtering?

@(^.^)@ Ed

WBX0510 board will be released soon, you can wait it.