EMRFD Message Archive 2023
Message Date From Subject 2023 2008-09-07 18:01:04 brian_vandewetter... EMRFD Signal Generator Extender fig 7.28 I built the Signal Generator Extender to go along with my recently
aquired 608E to provide coverage below 10Mhz. It seems to work and
the power output is right on at -20dBm up to 10Mhz where it tails off
to -25dBm. However, I am suprised at the amount of distortion in the
output signal which shows on my scope (See pictures in my photo
album). The desired frequency is the difference of the 19.66Mhz LO
and the signal generators output 21-30mhz. I understand that the low
pass filter should reject the LO, RF and sum frequencies along with
their harmonics, but perhaps my expectations of the low pass filter
is too high.
Should I work to improve the quality of this signal? If so, should I
look at impedance matching the RF input or is a better LP filter the
place to look?
Thanks,
Brian
KE7RME2024 2008-09-07 19:10:40 Allison Parent Re: EMRFD Signal Generator Extender fig 7.28 2025 2008-09-07 19:33:39 kerrypwr Re: EMRFD Signal Generator Extender fig 7.28 The circuit as printed should do much better than that; it's just a mixer.
Perhaps the collector tuned circuit is tuned to a harmonic, not the
fundamental.
Check all your work, including component values; I have adopted the
practice of checking all component values before use. AADE meter for
C & L; multimeter for R.
It saves a lot of work later!!
I've been doing it for some years and I now have trouble remembering
the standard colour code (that may be old age, though!!).2026 2008-09-07 21:13:37 bob bailey Re: EMRFD Signal Generator Extender fig 7.28 I've been doing it for some years and I now have trouble remembering
the standard colour code (that may be old age, though!!).
that's funny, I am starting to have the same problem.
bob
WB7DMX
2031 2008-09-08 03:53:29 brian_vandewetter... Re: EMRFD Signal Generator Extender fig 7.28 I'll go back and verify the values of the components. As far as color
codes go, I've not learned them yet. Too much else to think about and
the multimeter is so handy.
-Brian
2032 2008-09-08 09:12:32 bobtbobbo Re: EMRFD Signal Generator Extender fig 7.28 2034 2008-09-08 14:59:19 kerrypwr Re: EMRFD Signal Generator Extender fig 7.28 One thing that occurred to me; is the LPF output terminated in 50 ohms
for the test?
Filters should be terminated at each end in their design impedance; 50
ohms I assume here.
Termination in the high impedance of the 'scope might affect filter
performance. If you're not already doing so, terminate with a 50-ohm
resistor and apply the 'scope probe to that.2037 2008-09-09 09:56:15 brian_vandewetter... Re: EMRFD Signal Generator Extender fig 7.28 2040 2008-09-09 15:02:38 kerrypwr Re: EMRFD Signal Generator Extender fig 7.28 I think you have answered your own questions!! :)
Every little thing helps in the quest for improvement.
Grounding is important as you have discovered.
LO isolation is also useful; +7 dBm is a fair bit of power in the
context and any leakage may upset things.
The waveform will never be "perfect"; it's in the real-world nature of
mixers that there will be "oddities" but I thought the original
waveforms were poor; your later photos look much better.
I don't have anything the same as the Extender handy to test but I
have a mixer/amplifier that's almost the same as the Measurement
Receiver one; mine has a diplexer tuned for the 5 MHz IF.
I'm fortunate to have an analyser so I don't normally check
waveforms/distortions with a 'scope.
But I did so last night to see what mixer waveforms look like; photos
of the spectrum analyser screen and two 'scope displays (TDS320
digital & 465M analog) are in my photo album (kerrypwr).
They might give some indication of what to expect.
The test conditions were RF 10 MHz/LO 5 MHz. I also did some at
25/20; the waveforms were not as good but still OK.