RadioBanter - View Single Post

November 26th 03, 12:08 AM

(Fred McKenzie) wrote in message ...
I just acquired an HP 8640B and now I'd like to get rid of my HP 606A but I
need to go below 500 KHz.

Hank-

Recently someone mentioned that the 8640B was a UHF signal generator that had
digital dividers for lower frequencies. If that is true, you might be able to
add additional dividers to go lower in frequency. You might have to sacrifice
one of the existing bands, depending on how extensive the modification is.

73, Fred, K4DII

Yes, it uses digital divide-by-2 stages, but to get low harmonic
content in the output, the square wave is filtered by some pretty
decent filters. I don't know what the going price of things like the
HP3325A is, but that's a decent generator with very good frequency
resolution. For the low frequencies involved, you could also just use
a DDS, followed by a switched attenuator. A mixer down from higher
freqs is OK if you don't care about phase noise, I guess; add a LPF to
get rid of RF and LO feedthrough. You do miss one of the nice things
about the HP8640B, though, which is high output power. Getting back
to +20dBm with op amps shouldn't be too hard, but to keep SNR good,
you'd want to follow that with a step attenuator anyway (or take the
amplifier out of the circuit), probably, and not rely on just the 8640
output level to do the trick at really low levels.

Cheers,
Tom