OUT OF STOCK - A NEW VERSION IS UNDER DEVELOPMENT
DC-55 MHZ Up-Converter for your VHF receiver or SDR
If it happens you own an FRG-9600 or similar VHF/UHF multi-mode receiver or a DVB- T dongle (RTL-SDR) or anything else capable of receiving at 200-255 MHZ, this project is for you.
Adding this converter between an antenna and your receiver, you are now able to cover 0-55 MHZ, by just tuning your receiver on 200-255 MHZ .
This up-converter module, adds a very low-noise 200 MHZ carrier signal to the incoming 0-55 MHZ signals using a passive double-balanced mixer. It operates with +5VDC and has a mini-USB socket which when not connected, the converter is switched out and your receiver operates normally as before. By plugging-in the usb plug, it takes power and switches itself in circuit. In addition it features a high-gain Low Noise Amplifier (LNA) for the HF and 6m bands, which is manually switched in or out using a small jumper.
The mini-USB socket circuit has a fuse for protection and filtering networks to help greatly reduce or eliminate common-mode noise coming from the computer. Additional noise filtering is used before applying power to the low-noise high-stability 25ppm 200 MHZ crystal oscillator and to the LNA.
The Antenna input SMA connector is directly connected to the receiver output SMA connector using an RF relay's normally-closed contacts and a 50-ohms microstrip transmission line. This allows for minimum loss when the converter is out of circuit. No current is drawn and no harm in terms of losses exists when you need to operate your receiver without the converter. When DC power is applied (or when J1 jumper is placed), the RF relay connects the antenna to an m-derived 55 MHZ low-pass filter. Then, another similar relay is used to switch-in or out the LNA. The LNA is based on two parallel Gali-74+ amplifiers made by Mini-Circuits and offers about 20 dB of gain with 3 dB of Noise Figure.
Then, the HF signals are applied to the IF port of the double-balanced mixer ADE-1ASK. The mixer IF port has chosen instead of the RF port because it's frequency response starts at DC, while the RF port's starts at 2 MHZ. This makes possible to use this up-converter for VLF/LF too. As it can be seen on the schematic, with the LNA switched out, there is a DC path from the antenna connector all the way down to the mixer's IF port. With the LNA switched-in, the lower end of the frequency response gets raised to just under 70 KHZ. This is mostly from the transformers ferrite material used (BN-73-202).
The mixer's RF port now has many signals, coming out and in (reflections) of it. They are all the products of the mixing operation, the difference of the LO and IF frequencies, n-th order products, harmonics etc. including the desired sum, 200-255 MHZ ones. There is a lot of discussion published about terminating mixer ports and I have chosen to use a diplexer here. This 9-pole diplexer has two branches, one low-pass and one high-pass, centered on 200 MHZ with a Legendre response. The Legendre co-efficients makes a response like between Chebyshev and Butterworth. The high-pass branch passes the desired frequency band of 200-255 MHZ to the output connector with minimal loss while the low-pass branch burns the rest of the products with anything less than 200 MHZ on frequency, on a 50 ohm resistor (R1) so they won't get reflected back to the mixer. By using the diplexer, performance is greatly improved compared to other simpler designs.
The 200 MHZ signal for the mixer's LO port is generated in U2, an integrated low-noise high-stability HCMOS clock oscillator. One 5-pole low-pass filter and one 5-pole high-pass filter together with a T-type resistive attenuator are placed along its path. The LO signal leakage at the output connector is -50 dBm and if receiving VLF is of interest, a notch filter should be added in series to reduce it's power.
Fig. 5 Frequency spectrum at the output of the converter with one strong (-20 dBm) 55 MHZ signal at the input, and with the pre-amp OFF. The wanted signal (255 MHZ) is at the right (Marker 3) and has lost about 10 dB from its power (passive mixer, filters corner, cable). At the center is the LO leakage (Marker 2) and at the left is the image at145 MHZ (Marker 1).
Fig. 6 Frequency spectrum at the output of the converter with one strong (-20 dBm) 55 MHZ signal at the input, and with the 2x Gali-74 pre-amp ON.
The wanted signal is at the right (Marker 3). Now, it shows 20 dB of gain. At the center is the LO leakage (Marker 2) and at the left the image at145 MHZ (Marker 1).
Using the HF Up-converter
If the USB plug that provides the module with DC power is missing, (or is plugged-in but jumper JP2 is missing) then your VHF receiver or your USB dongle can be used just as before, as no current is drawing and the relays transfer your antenna to the dongle without losses and there are no other interference signals (like from the LO). When DC power is applied, the converter is activated, the 200 MHZ LO starts and if you tune from 200-255 MHZ you can actually copy DC-55 MHZ with very good results. The 70 KHZ - 55 MHZ LNA is activated by installing jumper JP1. The drawn current is now about 110-130 mA. The LO may not be tuned on 200.00000 exactly. It may be say 1 KHZ more. Most SDR software have provisions for this and you can declare an LO offset so your main receiving frequency be exactly what displayed. The way the frequency mixing is applied, it doesn't invert side-bands of SSB transmissions (LSB remains LSB and USB remains USB).
If you will never use the LNA, you can keep current drawing low by cutting the pcb trace under JP3 which powers the LNA. If you will need it again in the future, just add jumper JP3 from the top side of the PCB. I will prepare a good notch filter at 200 MHZ to help users of VLF very soon. Enjoy higher-performance at HF and if you still need better, make a pre-selector or use a high-Q antenna (magnetic loop).
The new Ready Built & Tested Encased Up-Converter
Added features on the encased upconverter:
Gas Discharge Tube at the HF antenna input for protection
Jumper to choose which antenna to select (HF or VHF) when the upconverter is out of circuit (pass-through).
Higher power common mode reject filter at the DC input, useful if your HF antenna ground is poor
Jumper to select internal 200 MHZ LO or external (for example from the external GPS Disciplined Oscillator)
DC power output for the matching GPSDO
Toggle switch for the HF LNA
Toggle switch for Upconverter in/out
Aluminum box for easier mount