[time-nuts] HP3325B as LO for a transceiver or receiver? Phase noise?

Mark Goldberg marklgoldberg at gmail.com
Wed Jan 2 01:33:55 UTC 2019


On Tue, Jan 1, 2019 at 5:31 PM jimlux <jimlux at earthlink.net> wrote:

> Has someone measured the reciprocal mixing noise contribution from the
> TCXO -  You've got measurements on the *output* of the transmitter using
> the various reference oscillators, and, of course, there's measurements
> on various bare oscillators and signal generators.
>
> Rob Sherwood has measured many receivers at
http://www.sherweng.com/table.html. I haven't been able to get together to
try different TCXOs, but conventional receiver performance is largely due
to the synthesizer design and the roofing filters. Andy, K3WYC's testing,
linked on my page, with simulated CW signals found little difference when
changing the TCXO in the receiver, but about 11 dB difference when the
transmit TCXO was changed. You don't want the guy with the cheap TCXO
transmitting a block over from you. That is a big reason people still buy
the cheap ones. They don't affect me, just the other guy.



> I suppose that's the sort of thing you'd get with a "strong signal next
> to weak" receive test.
>
> But the original question was "how good does it have to be"...
>
> That is, what numerical performance is needed for an HF transceiver? For
> CW or SSB? for one of the digital modes like WSJT or PSK31 ?
>

I can tell you that trying to use digital modes when there is someone with
a bad SSB transceiver near you on field day is not fun. Better transmitters
are always better, but how good it has to be depends on how it is used. I
haven't really done quantitative testing. My goal was not to make it worse
than the original manufacturer's performance.


>
> The propagation path itself has a fair amount of "phase noise" in the
> single digit Hz sort of bandwidth due to ionospheric effect, but you're
> looking a bit farther out as far as a undesired strong interfering
> signal rejection standpoint.
>
> Mark, your first set of plots show the output phase noise being the
> same, regardless of TCXO, from 100-1000 Hz, which is where I think you'd
> be concerned, even for CW. The CW signal itself is on the order of
> 100-200 Hz wide (depending on the keying waveform).  So you'd not be as
> concerned about the 10 Hz out phase noise..
>

I expect you meant 10 kHz, and yes, the receiver filtering is good enough
and the synthesizer bad enough such that the oscillator phase noise does
not matter that much. It is largely only important for the transmitter, at
least in this radio.


>
> And of course, if the strong interfering signal you're trying to reject
> with good receiver performance has crummy Tx noise that's another problem.
>
> We see this all the time when folks try to use mass market design
> approaches for space flight.  Your product may meet FCC requirements of
> being down 40dB or 60dB out of band/channel, but still be intolerable if
> you're radiating a carrier 1 mW (so your spurious emissions are at
> -60dBm), and we're trying to receive a signal at -150dBm in that
> adjacent band/channel.  Example, 802.11, BT or 802.15 transmitter
> spurious outputs in the  7.15 GHz deepspace receive band, especially
> into the omni receive antenna used in safe mode.
>
> This was the whole problem with LightSquared (or whatever their name is
> now) - 10kW transmitters in a band adjacent to GPS L-band.
>
>
Exactly the problem at field day or while working a DXPedition, where lots
of radios are close together. Elecraft has some very good synthesizers and
very low phase noise and they are used a lot in those situations. Of course
for an SDR reciever, oscillator phase noise is way more important.


On Tue, Jan 1, 2019 at 6:03 PM Jeff Blaine <KeepWalking188 at ac0c.com> wrote:

> Another option is to use an OCXO.  You have to go pretty nuts to get a
> modern frequency agile gadget to beat the spectral purity of a decent
> oscillator.  And here pretty nuts is probably an understatement.  If you
> only need frequency stability, the OCXO is a solid option assuming you
> can get one with the proper freq.


Yes, and for my measurements made with a Perseus SDR, I modified it to use
an external Wenzel OCXO. The A/D noise floor now is the limiting factor.

Regards,

Mark
W7MLG



More information about the Time-nuts_lists.febo.com mailing list