c-multiplier, real life
in [Design]
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From: Mike on 3 Jun 2010 02:41 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [...] > Mostly overkill for what's almost a microvolt! I wonder why he used > differential jfets. That's just throwing away 3 dB of noise > performance. He needs low dc drift. When you get down to 1uV on your breadboard, do show us the scope photos. [...] > John Mike
From: Mike on 3 Jun 2010 03:37 Mike <spam(a)me.not> wrote: > John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [...] >> I don't need nV for the c-multiplier here. But nanovolts are easy if >> you do can do narrowband tuning or synchronous detection. JW was >> trying to measure noise. > > Nanovolts are never easy. Again, your goals seem to have changed, but > you do not mention the new ones. > > Narrowband tuning or synchronous detection constrains you to sine > waves. Even then, you need good low-level preamplifiers. Your AM502 > won't be much good down to nanovolt levels. I had to review the Analog Devices AD630 Balanced Modulator for verification, but synchronous detection will only give a DC output. This is of little use when you are trying to trace noise and need to view the actual waveforms. [...] >> John > > Mike Mike
From: Jan Panteltje on 3 Jun 2010 06:59 On a sunny day (Wed, 02 Jun 2010 17:02:45 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <tjrd06hpdgiafmof0hq05devdqebcecd06(a)4ax.com>: > >I think I did all this right... > >ftp://jjlarkin.lmi.net/C-mult_bb.JPG > >ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG > > >John > LM317 has > 60 Db ripple rejection? Why bother with all this?
From: George Herold on 3 Jun 2010 09:23 On Jun 2, 10:12 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 03 Jun 2010 00:56:16 GMT, Mike <s...(a)me.not> wrote: > >John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >> I think I did all this right... > > >>ftp://jjlarkin.lmi.net/C-mult_bb.JPG > > >>ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG > > >> John > > >Not exactly. The depletion width modulation from the Early effect acts as > >a conductance from collector to emitter. The base current and voltage are > >not altered, and the shielding provided by the base region has no effect.. > > >This means the transistor collector-emitter can be modeled as a resistor > >in parallel with a capacitor. > > >In order to get substantial ripple reduction, hang a large electrolytic > >from emitter to ground. Phil uses a 10uF ceramic. I recommend using the > >10uF in parallel with a 3300uf low ESR cap. > > I know that the c-mult works at high frequencies. What I wanted to > measure is how it works at low frequencies. Re is pretty low, so the > output pole is in the KHz range if you use a reasonable cap. Plus Re > and the ESR make a divider. 3300 uF wouldn't fit inside my current > product. I'm using a 120 uF polymer aluminum cap and some ceramics. > > > > >The scope probe will not be sufficient to measure the ripple. The ground > >lead has enough inductance to pick up all kinds of noise radiated from > >the equipment and coax cables. > > The scope probe is measuring the input ripple, 200 mv p-p. Collector > lead. It works fine for that. And I'm signal averaging 64:1 on both > scope channels anyhow. > > > > >This measurement will need a coax connector the same as the ones you are > >using, with a very short coax to the preamp. > > The emitter output is via coax, to the AM502. > > > > >The AM 502 has 25uV noise. If you are planning on measuring 25nV signals, > >it will require (25e-6/25e-9)^2 averages, or one million. Since you want > >to find ripple much lower than that, it will take correspondingly greater > >averaging. > > As you can see, the signals are pretty big. They don't even need > averaging... it just makes them prettier. > > > > >The liklihood of drift during the averaging is very high, which will wipe > >out the results. So your equipment will limit you to a minimum detectable > >signal level, perhaps in the region of 250nV. > > >I find the leads in your layout are quite long. These will radiate > >signals and act as antenna. Also, soldering the coax connectors along the > >edge of the pcb means they will pick up the noise currents that are > >forced to flow along the edge of the pcb due to skin effect. This is > >surprisingly effective even at fairly low frequencies, say in the tens of > >KHz. > > Wild overkill at 400 Hz! > > > > >A better arrangement would be to solder the connectors directly to the > >copper near the signal. You might be able to bend the legs on the > >existing ones enough to tilt them up so the coax can be screwed on. > >Failing that, there are coax connectors with legs that can be soldered > >vertically to the copper. Or drill a hole and use a bulkhead connector. > > >When you start reaching decent ripple attenuation, radiation from the > >coax shields will start limiting the results. You will need better coax > >cables with 100% shielding. Or go to hardline. > > Not at 400 Hz! > > > > >Another problem is the reference voltage driving the base. When you > >finish making the ripple measurements, you need to find a way to supply > >the base with well-filtered voltage from the same supply as the > >collector. This will give an indication of the overall performance of the > >ripple filter. > > That's calculable. It's the transistor I'm measuring here. I wanted to > see if the LT SPice models were in the ballpark. Looks like they > probably are. There was some conjecture in a previous thread that teit > Early voltages were unrealistically low. > > > > >The filter in the base circuit will require farily large series > >resistance, which will give additional voltage drop that is dependant on > >load current, beta, temperature, and the phase of the moon. This is > >probably why Phil went with a MPSA14 darlington. > > A volt or three of Vce seems to improve rejection. So a resistor from > base to ground is good, if you can waste the voltage. > > > > >What you are trying to do is not trivial. Most people end up with a > >shielded box, low noise preamplifiers, and battery operation. > > I think these numbers are good; 140 dB would be tricky, but 66 ain't. > But if anybody wants to reproduce them, I'd be delighted. > > If I have any Darlingtons around, I'll try one of them. Any > predictions? When I tried a Darlington in a Cap-multiplier I found that it reduced the DC impedance of the filter. (As one would expect) but that there was more noise on the output. Something like 4nV/rtHz versus 1nV/rtHz with a 2N3904. I'm not sure what darlington I used... perhaps the MPSA14 or BC517. (Those are in my darlington parts drawer.) George H. > > John- Hide quoted text - > > - Show quoted text -
From: John Larkin on 3 Jun 2010 09:41
On Thu, 03 Jun 2010 06:37:52 GMT, Mike <spam(a)me.not> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 03 Jun 2010 01:52:17 GMT, Mike <spam(a)me.not> wrote: >> >>>Mike <spam(a)me.not> wrote: >>> >>>> Not exactly. The depletion width modulation from the Early effect >>>> acts as a conductance from collector to emitter. The base current >>>> and voltage are not altered, and the shielding provided by the base >>>> region has no effect. >>> >>>Sorry, this is not very clear. The collector-emitter capacitance may >>>be quite low, perhaps 400nF. This means the feedthrough will be small >>>until you get up to 10KHz or so. >>> >>>[..] >>> >>>> What you are trying to do is not trivial. Most people end up with a >>>> shielded box, low noise preamplifiers, and battery operation. >>> >>>Please see what it took Jim Williams to do a 775 Nanovolt Noise >>>Measurement in AN124: >>> >>>http://cds.linear.com/docs/Application%20Note/an124f.pdf >>> >>>Shielded box, low noise preamplifiers, and battery operation. >> >> Mostly overkill for what's almost a microvolt! I wonder why he used >> differential jfets. That's just throwing away 3 dB of noise >> performance. >> >>> >>>And that's only 775nV. I believe you wanted to see down to several nV. >> >> I don't need nV for the c-multiplier here. But nanovolts are easy if >> you do can do narrowband tuning or synchronous detection. JW was >> trying to measure noise. > >Nanovolts are never easy. Again, your goals seem to have changed, but you >do not mention the new ones. My goal was to better characterize the c-multiplier. I did it. > >Narrowband tuning or synchronous detection constrains you to sine waves. >Even then, you need good low-level preamplifiers. Your AM502 won't be much >good down to nanovolt levels. If the post-detector is narrowband enough the AM502 if fine. If I heeded more selectivity than the scope's signal averaging can handle, I could drive a spectrum analyzer or a selective voltmeter. But it's academic. I'm seeing signals at the emitter in the 100 uV and up range. I have tons of signal, not that tons of signal is what you want from a noise filtering circuit. John |