Does anybody here has any experience with FRA?

I'm designing a FRA(Frequency Response Analyzer). Maximum frequency that I need is about 1MHz. Now for multiplication of response wave to reference Sin(a) and Cos(a) I have two ideas, either use a very fast MDAC which I couldn't find a suitable one till now or use an analog multiplier.

Your helps are welcomed :)

Reply to
saman
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If I were you I'd use a pair of double-balanced diode-ring mixers e.g. Mini-Circuits SBL-1 or one of the newer siblings. Generate sin/cosine signals (e.g. I and Q), amplify, feed through a 50-ohm attenuator pad (say, 6 dB or so), and into the "LO" ports of two SBL-1 mixers. Similarly, amplify/buffer the incoming signal, split it and pad it, and feed to the "RF" ports of the two mixers.

Feed the "IF" port of each mixer to a 50-ohm buffer (e.g. grounded-base amp) and feed it through a diplexor/filter which has a bandpass as wide or narrow as you wish, then measure the resulting output.

[The padding and buffering is required to ensure that each of the three ports on the mixer "sees" a 50-ohm impedance to any signals coming out of the port - this is necessary for proper operation.] If I were doing this, I'd consider using an IF of 3.579545 MHz, as this would allow the use of an inexpensive filter using cheap NTSC colorburst crystals (I think there probably some resonant transformers pre-wound for this frequency, too), and it's high enough above your upper frequency limit that a simple low-pass filter prior to mixing would eliminate any image problem.

I imagine that you can do this with just one mixer and one LO signal... I don't think you need the full quadrature pair in order to simply measure the signal amplitude.

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Dave Platt                                    AE6EO
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Reply to
Dave Platt

The device I'm designing is supposed to work from near DC to up to 1MHz. DDS is used for very accurate frequency generation.

Reply to
saman

How near to DC? Single-digit Hz, fractions thereof, tens of Hz, hundreds of Hz?

These diode-ring mixers are frequently used in direct-conversion receiver applications, where the signal coming out of the "IF" port consists of audio frequency information.

The frequency responses of the three ports of an SBL-1 (e.g.) vary... e.g. the LO and RF ports are spec'ed to allow 1 MHz through 600 MHz, while the IF port is spec'ed as "DC to 500 MHz".

Your application would probably be best treated as a direct-conversion modulator / up-converter. Feed your test signal into the IF port (which has good low-frequency response), feed your DDS signal to the LO port, and take the output from the RF port and filter it and do a level-detection on it.

Another type of mixer/multiplier you could consider is the "H-mode" mixer, or related FET-based switching mixers. One interesting type uses a family of commonly-available analog bus switch ICs (it's even been done with 4066 CMOS parts).

You can design a mixer/multipler with this sort of switch which doesn't require transformer coupling... you can use a simple capacitive coupling / bias network to bias the incoming signal up into the center of the IC's voltage range. The lower frequency limit would then be set by the size of the coupling capacitor, and the impedance of the bias network and the load.

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Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Reply to
Dave Platt

See if you can find a copy of "Experimental Methods in RF Design" to borrow, it has a good treatment of mixers in direct conversion and other applications.

Then take a look at SDR- Software Defined Radio. ie a Tayloe mixer.

Then get a look at schematics for commercial LOCK-IN Amplifiers which often have very accurate mixing techniques be definition.

I personally like the idea of a phase detector or doing it with a microprocessor, but I have no idea of the spex you wish to meet.

Steve

Reply to
osr

Diode mixers are good for much higher frequencies than you need; probably a simple Gilbert cell is suitable for your purposes. It takes three matched pairs of transistors, and the output is high impedance, suitable for lots of IF filter inputs.

That kind of multiplier also supplies gain, if you were going to need that...

The operational circuits called 'analog multiplier' are overkill for a task like this. Switching multipliers (diode mixers and H-bridge switches) will require you to deal with odd harmonics somehow.

Reply to
whit3rd

lowest frequency is 1mHz(1 milli Hz). Let me explain a bit. In FRA frequency of response wave and reference wave is the same the only thing that differ is their phase. The mixer should not have any AC coupling. Also phase accuracy is critical here and especially two reference waves should have exactly 90 degree phase shift.

After multiplication result will be integrated. Any DC component can introduce a huge error here. One of other challenges that I have is how to remove DC component in such low frequencies. One solution that I thought about is to use a bias DAC for that purpose.

Reply to
saman

I have done a similar project with Lock-In amplifiers. I've used ADG202 analog switch there. Yes Lock-In Amplifiers are superb considering their S/N ratio. Lock-Ins are good for as low as 10Hz but I need even lower frequencies.

Reply to
saman

Gilbert Cell sounds interesting. One of characteristics that is very important in final accuracy is THD.

Almost all of you have suggested analog multiplication. How about digital multiplication using MDAC ?

I have played for awhile with mathlab and tried to determine what parameters can determine final accuracy. I tried to multiply two waves with the same frequency that had phase shift and integrate the result. I found THD can cause errors in measurement.

The reason that I also inclined toward analog multiplication is just that.

Reply to
saman

I just want to add this that though if output from DDS passes a low pass filter before going to an analog multiplication it improves THD significantly but because as I said it is absolutely critical that sin(a) and cos(a) reference waves have exactly 90 degree phase shift. so if I pass sin(a) and cos(a) from low pass filters after they came out of DDS there is a possibility that phase shift of the two waves may not be exactly the same because errors in components value. while this is really negligible because sampling frequency of DDS is about 100 MSPS(100 time more than maximum frequency that I need) and 3db knee is far from frequency of the wave but still in worst case it may introduce a few degrees of phase shift. If I don't use low pass filter after DDS for analog multiplication, then THD of both analog multiplication and digital multiplication are the same.

Reply to
saman

I was reading on the web last night about FRA's (by which you can surmise I am a complete novice.) Could you hit your circuit with white noise and look at the response? If you just look at the response you'll get only amplitude information. But, I think, if you also record the input noise waveform you can tease out the pahse information as well.

George H.

Reply to
George Herold

Multiplying D/A converter and a sine/cosine lookup table does sound like a good way to do the mixing; two issues arise, though. First, the D/A will lookup (maybe 1000) values, and at 1 MHz that means you need a new lookup every nanosecond. At 1 Hz it means you only update the value every millisecond. It's hard to scale this solution to a wide range of target frequencies.

The second issue, is that there is a glitch when the D/A converter updates, and you don't want to integrate the glitches; your integrators, then, have to be programmable to be 'dead' for a short period, or you have to characterize the switching transients. It can be made to work, of course, like by using extra converters so each 'dead' time is covered by an alternate unit that was updated earlier.

Reply to
whit3rd

That is not noise really, it may looks like noise but it is a very precisely generated waveform that consists of multiple frequencies. after applying this waveform the response is read and response of each frequency is extracted using FFT.

Reply to
saman

glitch of D/A converter is a very good point that may influence final accuracy. thanks for pointing it out.

Reply to
saman

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