Setting Rules for SDR Transmitters
By: Dan Maloney
The spread of Software Defined Radio (SDR) technology is a godsend for radio frequency enthusiasts. SDR-based receivers and transmitters have become so cheap that you might already have a couple of TV sticks lying around your workbench.
But cheap comes at a price, usually in the form of frequency stability, which can be daunting in certain applications, especially in amateur radio where a clean spectrum is paramount. Therefore, we are pleased to see Tech Minds addressing the frequency stability issue of SDRs by using a GPS-specified oscillator. The device utilizes the ADALM-PLUTO SDR transceiver and the precision oscillator from Leo Bodnar Electronics. This oscillator can be programmed to output a rock-solid, GPS-compliant signal across a wide frequency range. The transceiver has an external oscillator input capable of seeking 40 MHz, which is well within the GPSDO’s range.
Setting it up is very simple; just use an SMA to UFL jumper to plug the oscillator’s output into the SDR’s external clock input and adjust the settings of both the SDR and the oscillator. Of course, not all SDRs have an external clock input, so your mileage may vary. However, if your device is properly equipped, this seems like a great way to achieve accurate frequency, as the video below demonstrates how much cheaper SDR devices can be when frequency stability is addressed.
Like any good ham, Tech Minds is doing his work to keep his signal clean and on target frequency. His primary use case for this device is working with QO-100, the first geostationary satellite repeater in the amateur radio domain. We must say, as hams living in two-thirds of the Earth not covered by this satellite, we are just yearning for a geostationary satellite (or two) of our own to play with such things.
The Relationship Between LEO BODNAR GPSDO and Adalm Pluto SDR
Here, we look at how we use the Leo Bodnar GPSDO Mini with Adalm Pluto to fix any frequency drift.
Mini Low-Jitter Precision GPSDO Reference Oscillator (1 Port)
A new mini version of Leo Bodnar’s popular low-jitter GPS clock – designed for users on a budget or with space constraints while still offering accuracy and performance.
Programmable Frequency 450Hz – 800MHz
Stability 1 ppb 1E-9 <60 seconds lock time
1 SMA output connector
Ideal choice for use with SDR receivers
Like its big brother, the mini precision GPSDO reference oscillator can serve as a universal reference clock for most other instruments and applications that currently require a bulky and expensive rubidium atomic clock.
SDR-Kits recommends both versions as ideal frequency standards, providing external reference clock signals for the DG8SAQ vector network analyzer for frequency and frequency deviation measurements with accuracy approaching 1e-10, as well as frequency measurements in the mHz (millihertz) and even down to the uHz (microhertz) range (dF).
Additionally, the mini GPSDO can serve as an external clock for the SDRplay RSP2 or RSP2pro, using our GPSDO – RSP2 external clock cable.
Mini GPSDO Provides:
The long-term stability of the output signal is determined by the high precision of the GPS cesium reference, close to 1×10-12.
The short-term signal quality is defined by the internal TCXO clock source, providing high-quality, low-phase noise clock signals with sub-picosecond effective jitter.
The digital PLL allows the main output reference frequency to be set to almost any value between 400Hz and 810MHz.
The output signal is at 3.3V CMOS level with a characteristic impedance of 50 ohms. The output drive level can be adjusted.
Output Power Levels (measured at 10MHz, basic power channel):
+13.3dBm, drive set to 32mA
+12.7dBm, drive set to 24mA
+11.4dBm, drive set to 16mA
+7.7dBm, drive set to 8mA
Powered via mini USB from a PC, USB phone charger, or USB power bank. The USB connection to a Windows PC only requires a one-time setup.
How to Make High Precision Frequency with VNWA Thanks to Kurt Poulsen for preparing this report and sharing his expertise.
How to Set Up the Mini GPSDO as an External Clock Reference for VNWA 3EC: A guide for setting the mini GPSDO as an external reference clock instead of the internal VNWA TCXO during VNWA measurements.
Product Contents
Mini Precision GPSDO Module (73mm x 40mm x 17mm 0.05 kg)
GPS Magnetic Antenna with 5m Coaxial Cable and SMA Male Connector
USB Cable A to Mini B
12-month Manufacturer Warranty
Manufacturer Product Support Information
SDR-Kits Also Offers Accessories:
RG223 20cm Coaxial Cable with BNC Plug to SMA Plug Connector (GPSD – VNWA Ext Reference Cable).
GPSDO – RSP External Clock Input Cable + SMA Plug to BNC Socket Adapter
GPS Antenna 5m Long SMA Plug
SDR-Kits is an authorized distributor of Leo Bodnar Ltd Precision GPSDO Reference Oscillators.
It only requires a cheap GPS chip with a clock output of 10kHz or higher. Some GPS chips have the ability to reconfigure the frequency of the time pulse signal output from about 0.25Hz to 10MHz. The short-term jitter in this regard is very poor – really, really poor, but long-term stability is absolutely excellent. So you would combine it with a TCXO, which has very low short-term jitter and decent long-term stability. The best way to combine the two is to use a DSPLL (jitter attenuator, any part similar to AD9547/AD9549 or Si5397/Si5396).
For a circuit board running at 10 MHz to 750 MHz, you might need at least a 4-layer PCB to keep the signal as clean as possible. These costs can add up when you include a microcontroller and program it (which may require UART/SPI to configure the GPS TIMEPULSE output at power-up, as well as configuring the jitter attenuator parameters at power-up and possibly storing and updating long-term drift values), along with a high-quality metal enclosure considering initial production issues. Add your own 30-40% profit margin, plus a distributor’s 30% profit margin, (without them, the devices you sell worldwide would be much less, as they often add value by being a local filter/buffer between you and some major nightmare customers) – it’s easy to reach $150 without breaking a sweat.For most people, the real question is that the “fun” of creating their own device is more valuable to them than convenience.Those who frequent this place are more likely to have the knowledge and skills to build according to their exact specifications and needs rather than opt for convenience.
Part of the challenge lies in calculating the correct phase difference between GPS satellites, which includes relativistic compensation and some other interesting things. Deceiving GPS satellites is not hard… the hard part is deceiving the moving constellations of various satellites so that the target device parses out the desired values. What do you think about this approach to solving frequency instability issues? Feel free to comment!
【Ham DIY】Spread Spectrum SDR Receiver from 46MHz to 6GHz | Taking ADALM-PLUTO as an Example
