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Re: [Discuss-gnuradio] LFRX boards at low-VHF frequencies

From: Martin DvH
Subject: Re: [Discuss-gnuradio] LFRX boards at low-VHF frequencies
Date: Sun, 14 Mar 2010 15:55:10 +0100

On Fri, 2010-03-12 at 14:12 -0500, Marcus D. Leech wrote:
> Has anybody on-list had experience using the BASIC_RX or LFRX boards at
> low-vhf frequencies
>   (30-50MHz)??
BasicRX works when you add a corresponding bandfilter and a LNA.

Note however that the first niquist frequency is at 32 Mhz with a
default USRP1 with a 64 MHz ADC clock.

So signals above 32 Mhz will fold around to stations below 32 Mhz.
You can remove this effect with a bandfilter, but you need some cutting
space to do this.
With a very tight filter you can do about 98-126 Mhz or  66-94 Mhz or
34 - 62 Mhz or 0 - 30 Mhz.
But around the niquist freqs itsself +/- 32 Mhz  +/64 MHz, +/- 96 Mhz is
not possible.

If you use a different clockfreq for the ADC you can change the niquist
The USRP2 for example uses a 100 MHz ADC clock. With this you can do
about 0 - 48 Mhz wit a tight lowpass filter.
Running the USRP1 at 56 MHz will give you around  30 - 54 Mhz or 0 - 26
Mhz with the right bandfilters.

You can easily check this by running usrp_WFM_rcv.py on the commercial
FM band with a basicRX daughterboard. 

Note that there is a niquist freq at 96 Mhz with the default clock ( 64
Mhz). So if you leave out the bandfilter the stations above 96 Mhz fill
fold around on top of the stations below 96 Mhz.

I use a programmable clockgenerator with a USRP1 for experiments like

I haven't tried with LFRX. These use diffential amplifiers which have a
limited bandwidth. They will probably work up to 50 Mhz but not much
above that.

BasicRX works up to around 200 MHz. But also note that the higher the
frequency, the lower the sensitivity and the higher the phasenoise.

> I have a potential science project that could usefully be implemented in
> that frequency range.


> Cheers

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