UniSource FC-8300 - Frequency Counter Review

[olsenn]

I recently broke down and purchased the UniSource FC-8300: a professional, 3.7GHz, 9-digit frequency counter, which I obtained for $312 from TestEquipmentDepot (http://www.testequipmentdepot.com/unisource/frequencycounter/fc8300.htm). I just thought I'd share my thoughts on this unit so far.

This device is actually from a reference design which is also sold by several other companies, including B&K Precision (model: 1856D), and EZ-Digital (model: FC-3000). These are the exact same products right down to the User Manual, and as such the software (for remote access) is cross-compatible between vendors. Connectivity is achieved via RS-232, and example code is supplied along with a demo application which allows the user to set the operating mode, state, gate time, and other features remotely, as well as display the value that is being read on the screen. The UniSource software graphs out the frequency values for you in real-time, whereas the B&K Precision software allows this data to be stored to the hard drive in the form of an Excel spreadsheet.   

According to the datasheet, the default temperature compensated oscillator (TCO) has 1ppm accuracy (not including aging) and this is fairly accurate as measured using my FE-5680A Rubidium Frequency Std. The one gripe that I have with this unit is that while it does support an external 10MHz input (as any good counter should) via a switched BNC connector on the back, it only accepts inputs that are 1.5 - 5 volts RMS. Without external circuitry, my rubidium oscillator only puts out 0.7 volts RMS. There is also apparently other options available for the internal timebase (such as ovenized crystal); however, I could not find any information about ordering these options.

This counter features two inputs: a DC-coupled BNC (Input-A) which goes up to 100MHz, and an AC-coupled (Input-C) N-female which goes from 80MHz to 3.7GHz. Input-A has a 1Mohm input impedance and can safely accept input voltages of up to 200 volts peak-to-peak. Input-C (I have no idea why it's not called Input-B) is terminated with a 50-ohm impedance and can only accept 3 volts on the input (about 20 dbm). I have tested Input-A to its full bandwidth and Input-C up to 1.5GHz and it is accurate throughout these ranges.

As far as features are concerned, this unit has three modes of operation: frequency (uses reciprocal method (auto) for low frequencies), period, RPM, and Total. Note that only the frequency mode is available for Input-C. The gate time can be toggled between 10ms, 100ms, 1s, and 10s (an extra digit of resolution is made available to you with each increase in gate time). There is a Hold button to freeze the display, a button for manually enabling AC coupling on Input-A, a button to enable a 100kHz low-pass filter to reduce noise when measuring low frequency signals, and the input attenuation can be selected to 1X or 10X.

I did open the case of this counter to view the inner construction quality (you all know Dave's motto) but unfortunately I didn't have my camera with me at the time. What I can say is that it looks very nicely constructed with no bodge wires or cold solder joints, and the entire PCB is enclosed in a metal shield to prevent external EMF from affecting high frequency accuracy. The main microcontroller is an Atmel 40-pin IC in a DIP package, and there are plenty of other dedicated ICs for handling the work. I was afraid for a moment when I realized that one of the wires leading up to the Ext. reference was disconnected (I thought it broke off), but it is actually not connected by design (I tested the ext reference and it does work properly). The fuse can be accessed via the rear panel, as can the mains voltage switch (120/240). There is also a banana jack receptical for Earth GND (in case anybody needs that), and the RS-232 port as mentioned above.

Here is a photo of the front of the device for your enjoyment.

[olsenn]

Just one update; only 8 of the 9 digits of resolution are available in frequency mode (at 10s); the ninth digit seems to only be used for Total mode

[amspire]

Can you get extra resolution by letting the counter overflow, and letting it discard the most significant digits, or does it insist on some kind of autoranging? I guess you need a source over 10MHz to test that. If the gate cannot go longer then 10s, I guess the best you will get below 10MHz is 8 digits.

The reason the C input is called C and not B is that it is just following the convention of the higher end counters.

If you get a better counter, is will have two identical non-divided inputs A and B, so that as well as period, you can measure interval between A and B, frequency or pulse ratio between A and B and a host of other neat two input measurement.

The C channel is always the channel with a pre-divider so it can handle higher frequencies then the directly connected channels. It never has the smart capabilities of the A and B channels.

You are complete correct that all counters should have a 10MHz external clock input. It is really annoying how it was so often an optional feature - if you were lucky. The costs of providing an option is probably much more then just adding the ext clock sync input to all counters. It is probably not that hard to retrofit, even if it means disabling the internal clock totally and adding your own 10MHz clock board. I am sure there are probably plenty of pre-assembled modules on ebay, etc that would do a great job.

Richard

[olsenn]

Can you get extra resolution by letting the counter overflow, and letting it discard the most significant digits, or does it insist on some kind of autoranging? I guess you need a source over 10MHz to test that. If the gate cannot go longer then 10s, I guess the best you will get below 10MHz is 8 digits.

In Total mode (only available for Input-A), I can keep track of the clock cycles over a larger period of time (as long as I have a means to keep track of time separately). As you say, if it is a fast signal, the 9-digit counter will overflow and I will lose the MSD's; however, I can always read those values first in normal frequency  measurement mode. I'm not really worried about going through all that though; 8-digits are plenty for my purposes.