In our analog receivers the DC offset was measured, and a 18 bit ADC was used to trim the offset to keep it to about a millivolt from wherever it was set. Some external decoding required a fixed DC offset, and this was available from the front panel. We also had a 63 dB range in the output level control, digital with accurate .1 dB steps. This was on the Microdyne 700 and 1620/1670 series telemetry receiving equipment.
Tell me about it. ;-) The diversity combiner took the linear 0 to 5 VDC AGC signals to combine the two video signals into a single signal with a fixed output level as the received levels changed. The AGC system in the combiner had to have under 1.5 mV error after the 14 op amps used to set levels and feed them through the venerable MC1496 used as an analog multiplier.
Mine was the error amplifier in an ultra low noise RF synthesiser. The low frequency drift that resulted from the air currents resulted in phase noise from the oscillator in the .01 to 1Hz range. It was really hard to fault find.
d
My point about microwave is that without filtering you have aditional noise to deal with in the system. We offered IF bandwidths from 10 KHz to 20 MHz, and video amps with matching bandwidths to keep as much noise out of the received signal as possible.
The same receivers are used by NOAA for their weather sat downlinks. With the improved doppler compensation they are able to lock onto the sat before it comes over the hrizon five minutes faster than the old Harris equipment it replaced.
Ah! I see what you mean. Of course matching a receive filter to the necessary bandwidth means that you neither lose much-needed signal, nor let in unwanted noise. That goes for IF and baseband. As for RF, presumably you have a range of input frequencies to deal with, so you can't implement a channel-width filter there. But what you can do is a roofing filter and image filters.
I presume your Doppler compensation was predictive, offsetting the receiver by 17000MPH worth of frequency in anticipation of acquisition.
d
I forget how this got into this, but I once built a 20 bit, well I don't know how many bits, but I overlapped two 12 bit stages, and used discrets like a successive approximation register, and op-amps and got at least 20 bit offset correction. A/D D/A, but was interesting.
greg
A lot is still covered under several NDAs I had to sign, but the rest of it was making the system as phase clean as frequency stable as possible. Without that, the additional circuitry didn't stand a chance. There are only a few things you can do to compensate for the doppler effect, but that added to careful design and implementation works wonders. I also worked on the both the fixed and mobile earth station for Italy's space efforts. My title was production and engineering test tech, but a lot of the engineers ask why I didn't have a degree due to the way I not only found problems, but how to correct them. I got bounced all over the Ocala facility, where ever the current hot spots were. My boss told me that I wouldn't take "NO" for an answer. I smiled and told him, "I don't take YES if I don't believe them. ;-)
I had to clean up an older design RF synthesizer because of multiple engineering changes and parts going obsolete. The unit was inside a double shield, so it was quite temperature stable, but the uncased ceramic disk capacitor they originally used as standoffs was no longer available. They switched to a good quality microwave ceramic capacitor, and soldered them next to the large vias that were originally used to solder the caps to the board. This caused the phase noise to go through the roof. The fix was simple. Solder the holes closed, then solder the cap where it was in the original design. The PLL was broken into three band segments to reduce noise, and forward biased diodes were used to short out part of the inductor to raise the frequency. The rest of the phase noise was from the two RF output amps ringing due to a minor change in the layout. I was working to use a Maxim IC to replace the two amps when I was laid off in Sept. 2001. I can't remember the number right now, but the two outputs were 180 out of phase to reduce the current load variations. I also made some changes to the reference frequency traps to clean up the DC loop control voltage.
"Phil Allison" wrote in news: snipped-for-privacy@individual.net:
White noise has equal amounts of all frequencies. We hear the HF dominate because higher frequencies have more energy. (And because our ears are more sensitive to it).
The noise voltage of many op-amps is almost flat from 1K to 100K, but below 1 K it moves upward. I'm looking at the LF353 as it has a very low level of low frequency noise, or at least the chart shows that. The AD711 has 18 nvHz at 1 kHz but
60 nvHz at 1 Hz.greg
An octave is a doubling of the frequency, so it has logarithmic implications.
Tom
Ears are most sensitive around 1k, aren't they?
Tom
Tom MacIntyre wrote in news: snipped-for-privacy@4ax.com:
Probably. :) I wasn't thinking hard about that, I was just caught by the claim that HF dominates white noise in the audio band, when white noise is defined as being made up of all frequencies present with equal energy in each.
Hmmm...equal energy means that it would be more energy per octave at higher frequencies, right?
Tom
Tom MacIntyre wrote in news: snipped-for-privacy@4ax.com:
Yes, but that comes down to how you define the scale. The only reason, following from that, to say that HF dominates in the audio band, is because of expressing a log scale as lin, as pitch as opposed to frequency. If you say that the pitch notation is purely based on musical needs, and prefer the frequency scale for electronics analysis, surely you also have to discard with it the notion of HF dominance.
The reason the claim that HF dominates seems wrong to me, is I think a peice of string is its own length, regardless of whether we measue in inches or centimetres, let alone a nonlinear scale. White noise has a very specific definition, in which energy is equal across the spectrum, does not dominate part of it. It's out perception of pitch that does that.
Interesting about the string, because we can simulate musical octaves by folding a piece of string in half again and again.
Wouldn't a scale from, say, 100 Hz to 1600 Hz sound "interesting" if we used 100 Hz as the semitone, rather than 2^(1/12) x f? :-)
Tom
Tom MacIntyre wrote in news: snipped-for-privacy@4ax.com:
That would form the first 15 overtones of a natural harmonic series based on a fundamental of 100 Hz, and yes, it does sound cool. :) If you've got a synthesizer with either microtuning scales, or a resonant filter, you could set that up.
Aren't all Gallifreyans bar The Doctor now lost, since the Dalek war eradicated them ?
Arfa
"Arfa Daily" wrote in news:b7y5g.49$ snipped-for-privacy@newsfe4-gui.ntli.net:
Precisely. >:) You won't know which one I am. I have a lot of fun with this name, I can tell you. Besides, time is Strange, too strange for Daleks.
Lostgallifreyan spake thus:
Heck, took me about 3 minutes with Sound Forge. (Use the "Synthesis" tool.) Interesting scale. Sounds like the intervals get flat going up, to my ear. (Probably because it's untempered.)
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