Sound Card Spectrum Analyzer
Hi
A lot would depend on the phase noise of the source you were looking at. Without amplification the 346 is going to drop -170 dbc/Hz noise onto a 10 dbm source. Best guess is that you need 30 or 40 db more noise than that. As soon as you start adding amps keeping things accurate gets increasingly difficult. You need enough noise at the carrier, but you don't want the total RMS delivered to the mixer to be to crazy.
Calibrate with dual slopes, frequency response check with an RF carrier out of a signal generator. Does the job, works with basement level gear.
Bob
On Feb 19, 2010, at 2:35 AM, Gerhard Hoffmann wrote:
Bob Camp wrote:
If the noise is "known flat" it's a good way to check system bandwidth. Some means of checking response is indeed very necessary.
Not switching the preamp is indeed a good thing. The sound card does not have the range of the 3561, so with the sound card the beat note absolutely require a switch. The switch adds a second calibration step at audio.
To calibrate level, the levels of both the noise and carrier need to be well known. For a "basement system" the measure the slope as it crosses zero is likely more accurate. That of course assumes that you do the audio gain and response stuff properly.
Many of the free applications that are out there will put a tone out of the card and track it back into the card. That should at least provide a tone to work with. I also should be something that can be fairly easily verified. The issue of mixer output impedance is still a little tricky without RF noise loading.
One of those 346 A/B/C noise sources with 15 dB excess noise should be about right
to check the noise floor.
http://freenet-homepage.de/dl4yhf/spectra1.html
looks like a good FFT program.
regards, Gerhard
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Bruce Griffiths wrote:
Not necessarily, it depends on the phase detector circuit details.
What phase detector output termination network did you use?
Aren't BF862's somewhat noisier than 2SK369's at low frequencies?
SRA-3H because I had them, 49R9 in series with 3n9 for sum termination.
LC low pass with fairly large capacitance, high load impedance.
as recommended in Crawford, Frequency Synthesize Design Handbook page 11.
he cites
Fisher, M.C. "Frequency Domain Measurement Systems",
10th Annual Precise Time an Time Interval Applications and Planning Meeting
as the source.
I could not see much of a difference between NXP BF862 and Toshiba 2SK369.
BUT --- the BF862 has MUCH smaller input capacitance, so one could at
least
parallel 3 times as many of them for the same input load.
Now that I have some access to an Agilent signal source analyzer on a
good-will-base
my own phase noise measurement capability has dropped in importance, so
I can
concentrate in oscillators. I could re-use them as a clock for a digital
PN solution with
2* Inphi S&H, 2* state of the art ADCs and FPGA, which is more on my
home turf.
I think that the Wavecrests use the Inphi S&H, too.
Gerhard
Gerhard Hoffmann wrote:
Bruce Griffiths wrote:
Not necessarily, it depends on the phase detector circuit details.
What phase detector output termination network did you use?
Aren't BF862's somewhat noisier than 2SK369's at low frequencies?
SRA-3H because I had them, 49R9 in series with 3n9 for sum termination.
LC low pass with fairly large capacitance, high load impedance.
as recommended in Crawford, Frequency Synthesize Design Handbook page 11.
he cites
Fisher, M.C. "Frequency Domain Measurement Systems",
10th Annual Precise Time an Time Interval Applications and Planning
Meeting
as the source.
I could not see much of a difference between NXP BF862 and Toshiba
2SK369.
BUT --- the BF862 has MUCH smaller input capacitance, so one could
at least
parallel 3 times as many of them for the same input load.
Now that I have some access to an Agilent signal source analyzer on a
good-will-base
my own phase noise measurement capability has dropped in importance,
so I can
concentrate in oscillators. I could re-use them as a clock for a
digital PN solution with
2* Inphi S&H, 2* state of the art ADCs and FPGA, which is more on my
home turf.
I think that the Wavecrests use the Inphi S&H, too.
Gerhard
That termination should result in a quasi trapezoidal beat frequency
waveform for low beat frequencies.
The noise would be somewhat lower (as NIST found) if the 49R9 were
replaced by a short.
Most of the improvement would be due to reflecting the sum frequency
back into the mixer.
The SRA-3H is likely to have a somewhat lower output impedance than an
RPD-1 or similar.
In which case your preamp with the paralled BJTs may be close to optimum.
If an RPD-1 were substituted a preamp using a single SSM2220 should have
somewhat lower noise than if your preamp were used.
If the output impedance (as a function of input levels) for several
mixers were to be measured for various IF port terminations this would
be useful.
Alternatively one could always ask Minicircuits about the characteristic
for their mixers and phase detectors.
The Wavecrest models that correspond to the extremely detailed patents
that I have seen use relatively conventional TAC interpolators.
Bruce