The history of FVCs, basically and chronologically, goes as follows:
- a charge-dispenser made of a voltage limiter, a capacitor, and diodes;
- Two Op-Amps and capacitor
- Two Op-Amps and capacitor plus bleed resistor
- Cascading two or more fast Sallen-Key filters
- Phase-Locked loop
You don't say which frequency to voltage converter (FVC) you are using, or what it is based upon (LM331, LM131, AD650, ADVFC32, LM2907/LM2917 etc. There are many ways to accomplish frequency to voltage conversion - there are both analogue and digital methods, and within those camps there are again various ways of achieving the same thing.
If you want to see how the FVC came about then here is an interesting historical explanation, from basics: What's All This Frequency-To-Voltage Converter Stuff, Anyhow? It is written by Robert A. Pease, the guy who designed the LM131 for National, so this information is straight from the horse's mouth, so to speak. A PDF is also available from there.
I will attempt to summarize the article below:
Thirty years ago, a guy asked him if he could show him how to make a Frequency-to-Voltage converter (FVC), while he was working at at George A. Philbrick Researches. He designed a charge-dispenser made of a voltage limiter, a capacitor, and diodes. Evidently, it worked pretty well.
In 1964 he put a new version into the old Philbrick Applications Manual.
The first amplifier has a limited output voltage. The p-p voltage
across the capacitor is pretty well established:
V p-p = 2Vz + 2Vd - 2Vd
So, the charge (Q = C × V p-p) flows through the feedback resistor of
the second amplifier. The output voltage will be, on the average:
Vout = Rf × C × V p-p × f
A few years later, he got into the Voltage-to-Frequency Converter (VFC) business and at the same time, he came up with an improved circuit for an FVC (see figure 2).
The input comparator is set up to accommodate TTL
signals, but if you put a resistor from the + input to -15 V, you can
accommodate symmetrical signals; a resistor from the + input to ground
will cut down the hysteresis and let you handle small signals.
The real improvement in this FVC was the bleeder resistor, the 3.3 MΩ added to the right end of the capacitor.
After he left Philbrick, he joined National and designed the LM131 voltage-to-frequency converter3, using completely different ideas than any of the Philbrick circuits.
It used Q = I × T, rather than the Q =
C × V employed by all of the Philbrick ones. It didn't need ±15 V; it
could run on +15 or +30 or +12 or +5 V—much easier to apply. BUT, it
still had the same constraint when you used it as an F-to-V converter:
If you want low ripple, it's hard to get fast response.
Cascading two or more fast Sallen-Key filters
In 1978, he wrote an application note on how to improve the response time of an FVC—in the Linear Apps Handbook.
I showed how to cascade
two or more fast Sallen-Key filters to give reasonably quick response,
yet filter out the ripple at 24 dB per octave.
In 1979, he wrote another App Note showing how to use a phase-locked loop to make a quicker F-to-V converter, about 2 ms.
That's about 10
cycles of the new frequency—a further 20:1 improvement.
Fast clock and digital counter
Recently, a guy asked him how to make a 60-Hz FVC with quick response
and negligible lag or delay.
I told him that the standard procedure is
to use a fast clock and a digital counter. But the number of counts
collected during one period is linearly proportional to the period of
the signal, and you might have to do some digital computations to
convert that to a signal representing the frequency. Then I realized
that a "multiplying" DAC can be used to divide in a reciprocal mode.
He built it up and it worked. This Frequency-to-Voltage converter settles in one cycle of the frequency and uses only a small number of parts.
The digital logic generates a couple of pulses at the time of each
rising edge of the incoming frequency (you could use some kind of dual
one-shot multivibrator, but I didn't have any of those around). The
first pulse loads the data from the CD4040 into the DAC (the pulse
also disables the path from the clock to the counter to avoid any
confusion from rippling in the counter). Then the second pulse resets
The MDAC has storage registers built in, so the data from the counter
is fed right in to the DAC when the WRITE-2-bar pulse is applied. The
MDAC isn't connected in the normal way, with the variable resistance
in the input path. The fixed resistor is in the input, and the
impedance controlled by the Digital code is connected as the feedback
resistor. This permits the multiplying DAC to act as a divider, so the
reciprocal function is done neatly—not in the digital realm, and not
in the analog world, but on the cusp between them. (More on this in a
few months). The LM607BN was chosen for the op amp because you need
low offset. It's cheap, Vos is only 25 µV typical (60 µV max.), and
you don't need a trimmer pot.
I seriously recommend that you read the original article, as my abridged version has, unfortunately, had to leave out most of the salient technical facts.