An improvised 9 GHz real-time spectrum analyser with 64 MHz bandwidth using RX888, SDR Console and a downconverter

An improvised 9 GHz real-time spectrum analyser with 64 MHz bandwidth using RX888, SDR Console and a downconverter.

DRAFT a work in progress January 2025

Having worked with a  9 GHz downconverter for my phase noise analyser, I realised I could use SDR Console with an RX888 SDR to get a 64 MHz baseband. Adding a low-noise amplifier, a step attenuator, and some filters gives a powerful real-time spectrum analyser for about US$500.

I have a US$2000 Rigol RSN3015N real-time spectrum analyser, which only has a 10 MHz real-time bandwidth up to 1.5 GHz. Fully optioned to 4 GHz and 40 MHz real-time bandwidth costs US$6800. It is a very good instrument but expensive.

I work with digital amateur TV (DATV) using DVB-T with a bandwidth of up to 7 MHz. I have an earlier post on using a BladeRF as a  40 MHz real-time bandwidth spectrum analyser. This is another way to achieve a high bandwidth at microwave frequencies.

A real-time spectrum analyser differs from a conventional swept spectrum analyser. A swept spectrum analyser sweeps a frequency range and plots the amplitude at different frequencies. The big advantage of a swept spectrum analyser is that it can have a big range, covering several GHz. This is very useful for checking the presence of harmonics at many times the fundamental frequency. However, a swept spectrum analyser only gives the amplitude of a signal at the time the sweep passes. On the other hand, a real-time analyser has a limited range of tens of MHz but shows the instantaneous, real-time signal. This is very useful for signals that vary a lot, come and go or are partly masked by a strong signal. Both types have a place.

A downconverter, at a minimum, has a mixer, a local oscillator and a filter. Ideally, the mixer gives just the sum and difference of the RF signal and the fixed or tunable local oscillator (LO) and passes it at an intermediate frequency (IF). In an analog broadcast FM radio, the desired station on 103 MHz is mixed with an LO of 92.3 MHz to give a 10.7 MHz IF frequency and an unwanted sum frequency of 195.3 MHz. The unwanted sum frequency is removed with a 10.7 MHz low pass or band pass filter. The bandwidth of the IF is equal to the bandwidth of a single frequency, 200 kHz for broadcast FM. Downconverters typically use the difference frequency rather than the sum. IFs are used where filtering, gain and demodulation are more manageable at the lower frequency.

An alternative receiver architecture is direct sampling, where the desired signal is mixed with an LO at the same frequency and converted down to the audio band or baseband. Analog direct conversion receivers are uncommon as they have poor selectivity, allowing nearby strong signals to interfere with the desired signal.

Software-defined radios (SDR) can use either architecture, a downconverter to the baseband or direct sampling at the baseband.

The frequency response of direct sampling SDRs is limited by the sampling rate of the analogue to digital converters (ADC), typically from hundreds of kHz to tens of MHz. Due to the Nyquist sampling, the maximum frequency response is half or less of the sampling rate. The RX888 used here is a direct sampling receiver with a frequency range from near zero to 64 MHz. As such, it can simultaneously sample the whole HF and the low VHF bands. Once digitised, the signal processing is done with digital signal processing for filters, selectivity and demodulation.

Many common SDRs, such as the RTL-SDR and Adlam Pluto, use an integrated downconverter to baseband. Downconverters give a much bigger frequency response from around 70 MHz to many GHz. The baseband of downconverting SDRs ranges from hundreds of kHz to tens of MHz.

An external downconverter with a wide baseband direct sampling SDR like the RX888 allows the best of both architectures, with a practical and low-cost high-frequency range from near DC to 10 GHz and a wide bandwidth to 64 MHz.

My improvised real-time spectrum analyser comprises parts on hand from an earlier project that used a downconverter with a phase noise analyser in addition to the RX888. However, the components are low-cost and high-performance. Almost all parts were purchased recently from Ali Express. Ali Express is cheap, quick and reliable imported to Australia.

The RX888 Mark II is a low-cost direct sampling SDR costing around US$170.It is only used with the 64 MHz baseband receiver, not the VHF receiver.

I found the RX888 MkII very easy to set up and get working with SDR Console software. The Techminds YouTube video below explains the whole process. As the RX888 Mk II uses USB 3.0 Type C, it does not need the complication of a network connection, although it could be used that way. I use an Intel Core i5-14400 desktop PC without a graphics card and have had no problems running at 64 MHz bandwidth.

The main specifications are:

LTC2208 16bit ADC at 130 MSPS

Dual RF input, HF frequency range: 1kHz-64Mhz, maximum real-time bandwidth 64MHz. VHF frequency range: 64M-1700Mhz, maximum real-time bandwidth 10MHz.

0.5ppm VCXO

ATT adjustment range -32dB to 0dB

VGA adjustment range -11dB to +34dB

External 27Mhz reference clock support

3.3v software switched Bias-Tee HF/VHF independent control

ADC PGA Rand Dither software control

Support mainstream SDR software. For HDSDR SDRConsole SDR# SDR++

For setup, firmware and use with SDR Console, see: https://www.youtube.com/watch?v=WmaB4dIh99Y&t=236s&ab_channel=TechMinds

ADL5801 high linearity, doubly balanced, active mixer

The mixer is a module with SMA connectors using the ADL5801 that has a high linearity, doubly balanced, active mixer core with an integrated LO buffer amplifier to provide high dynamic range frequency conversion from 10 MHz to 6 GHz, costing about US$30. An active mixer is more straightforward to use compared to a conventional mixer. A higher-frequency mixer could be used if desired. While simple devices, mixers have all sorts of complications, such as images and other spurious signals as they are not perfect devices. I will return to this later.

Specifications:

Broadband upconverter/downconverter

Power conversion gain of 1.8 dB

Broadband RF, LO, and IF ports

SSB noise figure (NF) of 9.75 dB

Input IP3: 28.5 dBm 

Input P1dB: 13.3 dBm

Typical LO drive: 0 dBm

Single-supply operation: 5 V at 130 mA 

BG7TBL WB-SG1  1 Hz to 8 GHz wideband signal generator

The local oscillator uses a BG7TBL WB-SG1  1 Hz to 8 GHz wideband signal generator. costing about US$150. It uses a good-quality TCXO and has low-phase noise. The output is 0 dB and can connect to the mixer directly. It has no output filtering, so low-pass filters should be used for demanding applications to suppress harmonics. The signal generator is available with other frequency ranges.

For detailed specifications and reviews, see:

https://reeve.com/Documents/Articles%20Papers/Reeve_WB-SG1_SigGen.pdf

https://www.youtube.com/watch?v=wJM-a2IWrKw&ab_channel=TonyAlbus

Additional parts.

There are a number of additional parts that can be used depending on the circumstances.

Low-noise RF amplifier

Low Noise Amplifier 20dB High Gain  0.1MHz to 6GHz US$15

Step attenuator

2W SMA-KK 0-3GHz 0-30dB Step RF Attenuator US$30. This one only goes to 3 GHz, but I had it on hand. It may operate at higher frequencies. I do like the push-button attenuators.

Low pass IF filter.

The RX888 Mk II has a built-in 65 MHz low-pass filter, but I ordered one before I realised it. Another can't hurt.

Mini-Circuits Low Pass Filter SMA SLP-70+  Lumped LC DC - 60 MHz, 50Ω US$50 (not cheap but good).

SDR Console V3 on Windows 11.

SDR Console V3 is an outstanding program that supports a wide range of hardware. The setup of the RX888 Mk II is covered earlier in the device RX888 Mk II discussion section.

The program provides an up or down converter in radio definitions; just tick the converter selection box and then edit the parameters for the down converter LO frequency.