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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 and degrades the instrument.

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 (el cheapo) 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 for output voltages above 5V 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 (some instruments have the SM77H) 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 to 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 heat-sinks 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
I replaced the single dual output amplifier with two single THS3095 opamps. I also increased the +/- 12V supplies to +/- 15V to give them more headroom. The performance at higher output voltages and at higher frequencies is now much improved. Still not great, but much better, at least worth the small investment and trouble to do this. The best performance is still with 5V p-p and lower output voltages. I wonder what the higher frequency models actually deliver, if mine at 30MHz is already very poor.

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 at the maximum output.

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 separate windings, I took advantage of that feature, and while experimenting while building, I created two separate supplies with a full bridges on either one. This is not really needed, but diodes are cheap and you can avoid ground problems (using a star ground), 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. 

Because there is no separate winding for the 5V supply, I needed to tap that from one of the 12V supplies. Due to that, I also wanted to avoid the inevitable switching noise coming from the 5V DC-DC regulator going back into to the 12V supply as much as possible. Using a DC-DC switching regulator for the 5V supply poses no issue because this supply is further regulated on the main board to create the 1V2, 1V5 and 3V3 supplies for the digital components. The +/- 12V supplies are used to power the analog side with the opamps for the output section. Any noise on the 12V rails can 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 LM317 and LM337 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 tolerances of your 317 and the 337, (which are notorious), 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 approx. 20V and 5V at 500mA). I 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. Or, change the two-prong AC connector to the 3-prong version.

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 heat-sinks 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 heat-sink I used was too small. In hindsight, I should have used two separate and larger heat-sinks. 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 heat-sink 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.

After I raised the +/- 12V supplies to +/ 15V to give the output amps more headroom, the reduced voltage drop over the regulators decreased so much that the extra heat-sink kludge is no longer needed.

With all the mods done, I'm happy with the results.

I am actually most impressed with the precision of the counter function now. With my GPSDO now fully operating, I could finally measure the actual precision. When I measure the 10MHz output of the GPSDO, the counter with a gate of 10 sec. gives a result that is exactly 1 Hz too low. (9.999.999.00 MHz) Not a bad result at all for these modifications. It takes about 15-30 minutes of warm-up time to reach the highest precision.

 

UPDATE : January 5th, 2021

Using a GPSDO as the master clock

Now that my GPSDO project is finally coming to a conclusion, it is time to start to use that as the master clock for the FY6600, something I had planned all along. It took along time to get that project finished, you can follow that on other post on this Blog.

The trick to use the 10 MHz GPSDO is that you need to multiply the 10 MHz 5x to get to the required 50 MHz for the master clock. At the same time, I wanted to allow myself to use the FY6600 with the updated OCXO as well as the GPSDO. I designed a circuit and build a PCB for that Clock Generation circuit. There are a few gotchas to worry about so I made some provisions to experiment with the circuit, just in case. The major issues I expect to have is with the 50 MHz clocks and the selection gates. They may not be fast enough. I could not prototype this with these parts so I will need to build the circuit on the PCB.

Here is the schematic I'm working from.

The circuit is pretty simple. Top left is the SMA connector where the 10MHz from the GPSDO comes in. It goes through a buffer and then to the PLL that is set for a 5x multiplier. In the bottom half, you see the new place for the D75J that I need to lift from the FY6600 main PCB. The toggle switch that is shown in the middle will allow me to switch between the two sources. The three gates allow me to select one or the other. The question I have is if the 74HC02 is fast enough for these 50 MHz clock signals. 

I may not even need these gates, because both the PLL and the D75J have disable/enable inputs. They are both connected to the switch as well, so if push comes to shove, I can select either method and hopefully have it working. If I use the enable/disable pins, I may be able to connect both outputs together, without needing the 74AC02. I'm not sure how much they "tri-state" when disabled and if they influence each other.


 


The layout is above. The expert users with eagle eyes will see that I committed to a "cardinal sin" by using two vias between the TCXO and the buffer. The datasheet tells you to avoid that, but I took the easy (lazy) way. Mea Culpa Conner Winfield folks.

The major headache for me was to mount this on the back panel. I did not want to mount it flush to the back-panel, just in case I want to add a fan. I also didn't want to block the few cooling slits in the back.  The method I eventually decided on is a little cumbersome in order to mount it vertically. On the top left hand side of the PCB are the soldering contacts for a very small sliding switch. It just protrudes through one of the cooling slits. The bottom left pads are for an SMA connector. This is actually a male version with a nut, and it will be connected to a male-female panel feed-through SMA connector. The distance to the left is such that the PCB can be mounted vertical and is secured by the feed-through connector. I just need to drill a hole for the feed-through connector on the bottom from one of the slits. 

The solder pads on the right-hand sid 😁e are for the 50 MHz out, GND and the 3V3 supply. These short leads will go to the main PCB of the FY6600 where the original oscillator was mounted before.

I believe starting with the FY6900, it has a 10 MHz master clock, so in that case, you could feed the GPSDO signal straight to that input and you don't need the PLL clock multiplier. A short wire will bridge this device.

If you didn't upgrade the original SM77H or the CTS CB3 oscillator with the D75J, the solder pads on my PCB will not allow you to mount it on the foot print, you have to use three short wires. If I do another turn, I will add a separate footprint. The SM77H also has an enable/disable pin.

I build one board and found that the enable/disable for the PLL chip did not do what I expected. The output is always there. This means that you will have to use the the 74AC02 to switch between one or the other source. The other small issue I had was with the soldering of the D75J on the board. I could not get it to work. The pads for the footprint I designed where too small. I ended up mounting it the same way as I did on the FY6600 main board, by using tiny bend leads that went from the TCXO to the board pads. 

Mounting the PCB in the enclosure went as I expected. I drilled a 6.5mm hole in the bottom of the most left vertical slit to mount the SMA throughput connector. I used the most left hand vertical slot, because that will keep the board out of the way if I decide to add a fan, and the three wires to connect the pads for the TXCO on the main board to the new PCB will be very short.

With that modification, we're just about coming at the end of what can be done to improve this very inexpensive but otherwise great value product for hobbyists.

 

Next steps?

The next step for me will be to order a C6  mains receptacle and a C5 mains cable with an earth ground and properly "ground" the instrument with a 10nF/500V capacitor and in parallel two 1Meg Ohm resistors in series. At that time, I may also replace the 5V DC-DC switching supply board with a switching 7805 equivalent and mount that on the power supply board. If I'm bored and have nothing else to do 😁, I may even design a proper PCB for the power supply.


Enjoy!


10 comments:

  1. Hi Paul,
    Zie in je profiel dat je uit Nederland komt net als ikzelf. Heb de laatste dagen diverse projecten van je bestudeerd, waaronder de curve tracer en de diff probes. Las net ook over je aanpassingen van de FY6600. Uiteraard kom ik bij jouw blog uit omdat ik in precies zulke projecten geïnteresseerd ben. Ik vroeg me af hoe ik in contact met je kon komen?

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