Review: £10 multimeters - Now includes the new RS-14!

[Mark Hennessy]

As a frequency counter it works well with my signal generator up to around 18kHz then it reads low and above 21mhz the reading  jumps around, a two transistor rf amp restores the readout up to around 50mhz, I'd suspect with more drive then higher frequencies could be measured, a dedicated frequency counter would be more practical though.

Curiously, my AN8002 is considerably more sensitive than my AN8008:

AN8002:
The sensitivity is very good. At low frequencies (<100kHz), it's about 20mV RMS (sine wave). At 1MHz, it's roughly 40mV. At 10MHz, you need just over 1.2V, and at 30MHz, 1.5V. As mentioned, I managed to get to 80MHz with the +17dBm (1.6V) from my RF generator, so it's interesting that the response essentially plateaus after the initial rise between 1 and 10MHz. Unless I've missed it, there's no specification given for sensitivity.

AN8008:
This is another one that requires the signal to cross 0V when measuring frequency. I found that the sensitivity was rather less good than the AN8002. At low frequencies (<1MHz), it's about 600mV RMS (sine wave). At 10MHz, it's roughly 1.4V (the AN8002 was 1.2V), and the highest the +17dBm (~1.6V) from my RF generator would get me was 80MHz.

Both those were in the dedicated "Hz" function though - they obviously differ on the AC Volts function. The AN8008 is less sensitive in this mode, thanks to what appears to be an auto-ranging bug:

There is a bug with measuring frequency on the voltage ranges. You need a minimum of ~1.7V to get a reliable reading, whereas the AN8002 works down to about 50mV, give or take, on a 1kHz sine wave. With a lower input voltage, if you use the yellow button to cycle back to AC, then press it again to get Hz, you get do a reading for about 3 seconds, then it changes to 0Hz. This works reliably down to about 100mV. I think what's happening here is the meter is erroneously auto-ranging higher voltage range, rendering it less sensitive.

But ultimately, I agree that a dedicated frequency counter is the best bet, especially if you can drive it from the Y-out of your oscilloscope if it has one (I wish more 'scopes had that). It's definitely a "nice to have" in a multimeter, and the fact it goes beyond a few 100s of kHz is quite surprising, given the way the signal is being handled internally.

Apparently some samples of the AN8002 don't have Hz and duty cycle on the Volts function. Do any of yours fall into this category, Trev?

Cheers,

Mark