Home Brew 10MHz Doubler-Distribution Amp

I completed this project in about late August 2012. What drove this project was the need for a 10MHz reference for my HP test equipment. I have a URQ-10A Frequency Standard which produces 5MHz, 1MHz, and 100KHz but no 10MHz. I originally bought a General Microwave 550 WWV receiver because it produced a 10MHz reference out. However, this meant I had to have the WWV receiver on when I needed 10MHz and it only provided a single 10MHz output. It occured to me I could double the 5MHz, amp it, and then power divide it for distribution. Thus the project was born.  I spent quite a bit of time breadboarding and testing different circuit configurations. I found out just how hard it is to supress harmonics while testing and breadboarding. The one thing I learned was you never really get rid of harmonics. They just naturally exist. Below is the final design I settled on.

5MHz Doubler - Filter
For the 5MHz doubler I decided on a balanced bridge mixer based on the fact that  a balanced mixer would cancel out the 5MHz component at the output. For the mixer diodes I used a match set of HP 5082-9496 Shotkey diodes. On the input I used a 5MHz series resonant circuit to provide some amount of bandpass and suppress the harmonics of the 5MHz input. Since balanced was the prime factor in this circuit I needed to change the input from single ended to balanced through a transformer. The input transformer is wound on an FT43-50 toroid core (all the transformers in this project were wound on this core material). Input side was wound for 5MHZ at 50 ohms. Secondary to the balanced mixer was wound for 5MHz at 200 ohms. 200 ohms was selected because that was the forward conducting impedance of the bridge. Again I was trying to achieve balance. I wound a third winding at 50 ohms on the input transformer that I would use to monitor the 5MHz input. For the output of the bridge I needed another transformer wound for 10MHz at 200 ohms to keep things balanced. The next thing I needed to do was filter the mixer output for the 10MHz and get rid of (suppress) any harmonics and any other mix products that may be present. The second harmonic of 20MHz was quite strong after mixing so I started with a 10MHz series circuit to initially get some amount of bandpass filtering and followed it with a 20MHz series resonant trap. I then followed that with my primary means of filtering which was done with 10.7MHz bandpass filters that I retuned to 10MHz. I chose the Xicom 42IF-222-RC transformer which was built as an IF transformer for FM transistor radios. They retuned just fine for 10MHz and worked well. I used 3 of these transformers in parallel to try and get tight skirts on the bandpass filter. They had an input and output impedance of 300 ohms so I wound the secondary of the bridge out transformer for 10MHz at 300 ohms to keep impedance matched as well as changing back to single ended operation. I followed this bandpass circuit with another 10MHz series resonant circuit before converting from 300 ohms back to 50 ohms using another toroid coil transformer. At this point I was measuring 10MHz at a -17dbm with harmonics at 51 - 59db down. I was ready to move onto amplification.
Initially I tried breadboarding discrete amplifier circuits but in the end settled on the Mini Circuits ZFL-500N wide band amp. It has a gain of 20db and worked very well. Better than anything I could have come up with. With a -17dbm input I was getting a measured +3dbm out. Harmonics were 45 - 53db down at this point. However, this was not enough signal to drive a power divider and have any kind of sufficient signal to drive a test equipment ref input. So I needed to amplify some more. I first tried going straight into another Mini Circuits amp but 1) the +3dbm input was pushing the second amp into overdrive creating all sorts of mixing products and 2) because it's a wide band amp it was amplifying all those harmonics and distortion mix products way up in level. I needed to 1) do some more filtering on the output of the first amp and 2) knock the signal level down a bit to prevent over driving the second amp. For filtering I first used a Low Pass T Filter with a knee at 11.5MHz. I followed that with a 5MHz trap still trying to get rid of the original 5MHz input. Then I followed that with an 8db pad to further help knock the signal level down before inputting to the second amp. At the input to the second amp I now had a -6.6dbm 10MHz signal with harmonics 54 - 59db down. Output from the second amp was now a pretty clean +13.8dbm with harmonics at 47 - 51db down. I was very happy with this output and determined that I now had sufficient level to drive a power divider.
For a power divider I used a Mini Circuits ZBSC-615 6-Way Power Divider. One of the outputs I sent to the front panel and the other 5 outputs are on the rear of the unit. All unused ports must be terminated in 50 ohms to keep everything right. I put in a T-connector between the second amp and the power divider  to tap off some signal so I could monitor the 10MHz output at the front panel. I had outputs of +3.8dbm at each port with harmonics 47 - 48 db down. I was very satisfied with this output and it was at a sufficient level to drive a 10MHz ref input on test equipment.
I wanted to be able to monitor 5MHz input, 10MHz output, and the +15v supply. I had a nice little meter with a 0 - 10 scale on it and a full scale deflection of 100ua. I wanted the 5MHz to read midscale at 5, the 10MHz to read full scale at 10, and the +15v supply to read at midscale since there was no 15 on the scale. For the 5 and 10MHz I simply rectified and filtered a sampling of the signal to get DC. For the +15v supply it was a simply matter of resistor voltage division. I tapped the 5MHz at the input transformer with an additional winding on the input transformer. For the 10MHz I tapped off the output line to the power splitter. I put adjustment pots in  each of the legs to adjust for the proper reading on the meter scale. Worked very well.
 Power Supply
Power supply is a traditional LM317K 3-terminal adjustable regulator. It needed to provide +15vdc primarily for the amplifiers with a total of 200ma load current. I built the power supply in it's own chassis to isolate it as much as possible from the signal circuits.


Here's some shots of it built. I built the RF and PS sections in separate chassis and them joined them together. Mounting then to the front panel stabilized the whole assembly.  I wanted to keep the RF section isolated. I used all SMA connectors to keep size down. This was my first extensive use of SMA connectors and I really like them. The Doubler - Filter assembly and the filtering assembly between the two amps were fabricated using double sided PC board.  On the Dblr-Filter I sectioned off the input, bal mixer, and the post filtering circuits with double sided PC board as well. You see the SMA T-adapter where I tapped off the 10MHz line for monitoring.
Just some shots of the power supply. It does end up totally closed up with no special ventilation. I figured there's plenty of aluminum there to sink off any heat. In operation it gets pleasantly warm and that's about it. Here's some pictures of it complete all closed up. Front and rear views. I like to have a 10MHz output on the front panel as it's nice to be able to easily grab 10MHz for something I may be doing rather than reaching around the back to make a connection.

Finally here it is taking it's place in the shop Frequency Standard Rack 

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