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Sunday, September 1, 2019

Upgrading & Tuning my FY6600 Waveform Generator

For a while, I had contemplated to upgrade my FeelTech FY600-30MHz Dual Channel Waveform Generator.
This is a very interesting instrument that has specifications that would be comparable to instruments hundreds of Euro's more expensive. Unfortunately, the manufacturer of the design took some short-cuts to penny-pinch some more profit out of the device, and that causes some problems.

Initially I didn't care too much, the instrument performed very well for me, but when I started to dig into high precision oscillators and the likes, see my other posts, I needed a higher precision counter function. This can be rather easily done by replacing the main oscillator but I also wanted to see what other people did and what else was possible.

There is a huge number of postings about this instrument on this blog : eevblog.com , after reading through all of them (yes really - it took me three days) I decided to tackle the three most important issues.

These are four main issues reported by the many users on the forum.
  1. A very poor switching power supply with no earth ground. This could put an AC voltage as high as 1/2 of the main voltage on the BNC outputs. The current is very little though, and is caused by the leakage of a capacitor, but still.
  2. Distortion of the waveforms at higher amplitudes. This is caused by the selection of rather inexpensive and older design opamps. Interestingly, there are provisions on the PCB to install much better opamps.
  3. A rather poor main oscillator with poor precision and significant drift. The drift is mainly caused by the fact that the oscillator is way too close to three voltage regulators that get pretty hot.
  4. The attenuation circuit in the output section has an incorrect resistor value.

4. The attenuation issue
At this moment I'm not going to address that problem.

3. The oscillator
The recommended replacement oscillator for the CTS 50MHz 50ppm CB3 version is the model D75J from Connor Winfield. This is a surface mount temperature compensated crystal controller oscillator (TCXO) in the 50Mhz version, calibrated at 25 degrees at 1ppm and a stability of 2ppm. Incredible precision and stability! Because the D75J is a temperature controlled oscillator, the effects of the nearby regulators are pretty much eliminated. This is especially important if you use the FY as a counter.

To reduce some heat hot-spots from the three on-board regulators near the oscillator, I added two small "sticky tape" heat-sinks (Raspberry Pi type) on top of the three regulators. The temperature of the heatsinks after more than two hours is now about 46 degrees C, and more evenly spread.

After I installed the new TCXO, I measured the precision against a calibrated oscillator.  I measured the frequency of this oscillator to be 5,000,001.7 Hz. This is very close to the calibrated value of 5,000,002.1 Hz, but that factory calibration was eons ago, although the oscillator was never used until very recently (see my other post: Frequency Generator with Fast Edges). Either way, that is a remarkable precision and the reading over a few hours only changed 0.1Hz. The heating of the temperature inside the FY enclosure no longer seems to have an effect. The next step is to measure the result with a GPS Disciplined Oscillator (GPSDO), a project I'm currently working on.

2. The opamps
The two replacement THS3095 opamps are ordered. I'll update this section when I have them installed.

1. The power supply
The power supply has been talked about many times on the forum, with several options offered, but there are only a few examples for a replacement.  I decided to design a completely new supply, that would let me adjust the +/- 12V outputs to +/-15V as well. The new output opamps need this voltage to display the waveforms without distortion.

Going through my parts collection, I picked a 24VA block transformer that could be mounted on a protoboard. The transformer is a little too heavy (in VA) with 2 separate winding's of 12VA each, and an output of 15VAC. Both secondary winding's are fused with 0.8AT PTC fuses. The primary has two separate 115V winding's. Because there are seperate windings, I took advantage of that feature, and created two separate supplies. The advantage is that you can use full bridges on either one, saving on the size of the buffer cap, and you can avoid ground problems, especially easy to do when you are using perf- or protoboard instead of  a real pcb layout. I tied the supplies together at the connector going to the main board, not anywhere else. I also wanted to avoid the inevitable switching noise coming from the DC-DC regulator going to the -12V supply as much as possible. Any noise on the 12V rails can/will make it to the output signal. After measuring the final results, I added another 330nH inductor to the plus input of the DC-DC convertor, to prevent more switch noise from being injected back into the 12V supply. This added inductor is not shown on the circuit diagram below.

The weight of the transformer will prevent the instrument from sliding as much, because the instrument was so light before, a bonus.

I decided to use the LM318 and LM338 voltage regulators for both the +/- 12..15V analog supplies. The only specialty in this circuit  is around the 10-turn trimmers, because they are China quality, and therefore cannot really be relied upon. The worst case is when the runner looses contact, creating a much higher output voltage then you intended, and this could potentially blow up the output amplifiers of the FY6600. The way I selected the adjustment components is such that you can get just below 12V and just above 15V. Depending on the toleration's of your parts, you may have to adjust a resistor value here and there. I bread-boarded the circuits before I mounted them, just to be sure.

For the +5V supply used for the digital circuits, I selected a simple DC-DC Buck convertor because doing that with normal linear regulators would create too much heat (burning off the difference between 20V and 5V at 500mA). I specifically did not want to add a fan, with all it's generated high frequency switching noise inside the box. I also reused some parts from the old supply, specifically the line filter and the 5V choke.

My unit is intentionally still floating. This can be fixed easily with a BNC/USB connection to another earth grounded instrument. If I change my mind, I can still add a 4mm connector to the back, tie that to the power ground and make a connection to earth ground with a test lead that way.

Here is my design:



And here is the unit with the supply installed:



Just above the upper left hand corner of the transformer, you can see the two little heatsinks I used on top of the three regulators to disperse their heat. Just above it you can see the metal can oscillator that I'm going to replace. The person that did the layout of the board had no clue as to the heat effect of the regulators on the stability and precision of the oscillator. 👿

I did not have room on the protoboard for the DC-DC convertor, so I used double sided sticky tape to put it on top of the transformer and moved it away from the analog section of the main board. These DC-DC devices are notorious for their switching noise superimposed on the output voltage. Using the 10uH inductor from the old supply and a few capacitors on the output reduced the 25mV ripple at about 1MHz to about 7mV, which should be OK.

When I started the testing, I had to resort to a little kludge because the heatsink I used was too small. In hindsight, I should have used two separate and larger heatsinks. Doing that now would cause some major and ugly surgery. Bolting another one on it is now better with the supplies at +/- 12V, it will be significantly cooler when I change them to +/- 15V. (less of a voltage drop over the regulators) At +/- 12V, the heatsink is now just below 50 degrees C. After more than two hours, the temperature inside the case is about 35 degrees C, at a room temperature of 27 degrees C, which is a bit warmer than I like but fine for now.

More later...


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