Crystal Substitute Project:
I am currently testing out the QRP Labs ProgRock2 programable TCXO board (PR2) as a substitute for crystals in our ATV equipment. They seem to work well in AM and FM analog voice transmitters, but I need to take a look at the phase noise close in sidebands and since it puts out a square wave, how the harmonics affect the final output. Some just add a lowpass or bandpass filter to make it into a sine wave. Check back from time to time.

This application note describes a low cost 4 channel TCXO substitution for crystals in our P. C. Electronics ATV gear. Most all our ATV transmitters have the same oscillator circuit using 5th overtone series resonant HC49 or HC50 crystals. There are very few crystal manufacturers these days and those that are around dont make the 5th overtones in the greater than 100 MHz region. Small temperature compensated crystal oscillators (TCXO) have been around, but the phase noise and spurs have not been good enough not to be seen in the video.

QRP Labs came up with the ProgRock2, an $18 board that has 8 selectable binary coded decimal (BCD) channels and can be programmed with a PC for any frequency from about 3.5 kHz to 200 MHz. Frequency accuracy is .25ppm (that is a little over +/- 100 Hz at 70cm). You could program all 8 channels and use a BCD rotary switch to select them, but for most ATVers, the 4 most used frequencies can be individually selected with a single pole standard rotary switch - Common goes to ground, position 1 426.25 no connection, position 2 427.25 Bank 0 pad, position 3 434.0 Bank 1 pad and position 4 439.25 Bank 2 pad. The other 2 TCXO outputs are not used and are selected off.

P. C. Electronics Transmitter Crystal Frequencies:
439.25 MHz = 109.8125 MHz crystal
434.00 MHz = 108.5000 MHz crystal
427.25 MHz = 106.8125 MHz crystal
426.25 MHz = 106.5625 MHz crystal
421.25 MHz = 105.3125 MHz crystal (requires VSB filter on the TX output).
The 33cm transmitters are divided by 8 and the 23cm are divided by 12.
For other transmitters, downconverters and receivers, find the crystal frequency as determined by the formula on the respective equipment data sheet or manual.

Below are photos taken off the screen of a TinySA spectrum analyzer connected through 50 Ohm step attenuators and then a 1K resistor in series to the ProgRock2 Clock 0 output for these tests. The CLK load can affect the phase noise and current draw.

Square wave harmonics referenced at -30 dBm and 108.500 MHz (F). First odd harmonic at 325.5 MHz (3F) is only down 10dB which is typical for square waves having predominantly odd numbered harmonics. A low pass or band pass filter can be used to reduce them or convert to a sine wave. The second harmonic is down about 33 dB so it will not help the first doubler following the oscillator.
Full sweep width = 1000 MHz and 100 MHz per division starting at 0 MHz.

Close in phase noise is down more than 50 dB.
Full sweep width = 108.4 to 108.6 MHz and 20 kHz per division.

Phase lock loop spurs at +/- .5 and 1.5 MHz on each side of the 108.500 MHz fundamental frequency. Spurs are down about -47 dB from the 108.5 MHz fundamental.
Full sweep width is from 106.0 to 111.0 MHz and .5 MHz per division.

Programming the ProgRock2

Download and install the PuTTY terminal emulator. For most Windows 10 and 11 machines, that will be the 64-bit x86 MSI installer. Download and read the ProgRock2 manual.

Best to program the ProgRock2 before soldering anything to it or mounting on the transmitter board. You cannot have the USB programming cable connected to the ProgRock2 board and computer at the same time its +V pad is connected to the transmitter board. Careful not to place the board on or near any thing conductive that could make contact with the exposed board. Make sure the USB micro / USB-A cable is a data cable with all the wires vs. one that only has the +5 and ground for charging a device. QRP-Labs has one you can add at the time of the ProgRock2 purchase.

Plugging in the ProgRock2 board into the computer USB COM port with the USB cable will power the ProgRock2 up and allow finding the COM Port number. To find the COM port number, right click the Start Icon, click Device Manager, double click Ports (COM & LPT).

Open PuTTY. Click Session in the upper left corner. Under Connection Type, select Serial. Above it, under Serial Line, enter the COM Port number (it was 3 in my case).

Click on Window in the left side column, then set the Size of the Window to 80 Columns and 24 Rows. Then click Open. You should now get a black screen.

When the black screen comes up, hit Return (Enter) and the ProgRock2 programming screen should come up. Enter frequencies in Hz. For example, in Bank 2 in the first column (CLK 0 output) for transmitting on 434.0 MHz, enter the 108500000 crystal frequency. Verify you have all 9 numbers, including trailing zeros, for a frequency in Hz in the 100 to 200 MHz range.

On the above screenshot of the ProgRock2 frequency programming screen using the PuTTY terminal emulator on a Windows 11 laptop, the 3 cell columns correspond to the CLK 0, 1, 2 output pads. The rows, listed as Banks, are selected by grounding the Bank 0, 1, 2 pads in BCD format. All frequencies entered must have trailing zeros down to the Hz. All unused Bank cells in all 3 CLK (Clock) output columns need to be zero for no output if accidentally selected and to minimize current draw. Mine drew 48 mA with only CLK 0 output used.

The KEY to cursor movement is at the bottom of the screen and may take some getting used to. Play with it until you get the hang of it. Never, never, never hit f (See page 16 in the manual). When you think you have all frequencies entered correctly, punch s to program the board and save. You can use a FM broadcast receiver placed near the board to verify success. You should hear a carrier around 106.5 on the dial if you put 106562500 in Bank 0. The frequencies are kept in the boards non-volatile memory until written over later if you made a mistake or make changes,

Preparing and Mounting the PR2 Board

The top side of the PR2 board is easily identified by the USB jack and larger IC. All the wired connections will go to the pads on this side of the board and should be done before mounting. +V power is supplied through a 78L08 8V regulator so as not to over heat the on-board 3.3V regulator when connecting to 13.8V.

The bottom of the PR2 board has two ground pads at the end opposite of the USB jack side that can be used for mounting to the ground plane of the transmitter board near the crystal socket. Before applying power, confirm with an Ohm meter that the +V and CLK 0 pads on the other side are not shorted to ground.

Partial schematic of a PR2 crystal replacement board with a P.C. Electronics ATV transmitter. The PR2 board is powered through a 78L08 regulator to keep from over dissipating the on board 3.3V regulator. The CLK 0 output connects to the oscillator base through a 100 pF capacitor. Remove the 10 pF feedback cap between the 1st doubler base and oscillator emitter (not the one to the collector). Remove the 10K oscillator base bias resistor. Add a 100 pF from the oscillator emitter to ground.

PR2 mounted on a TXA5 ATV Exciter board.