The Muntjac SDR

The Muntjac SDR was designed by Brian G4EWJ based on the same chip as the commercially-available Caribou SDR. The Muntjac is a transmit-only SDR for DVB-S2 DATV that uses an RP2040 micro-controller to provide a USB interface to the Portsdown 4.

Specifications

The Muntjac SDR only operates with output frequencies in the following ranges: 390 - 510 MHz, 780 - 1020 MHz and 2400 - 2485 MHz. Individual examples may operate slightly outside these frequency ranges, but not significantly. For our use, this means that the Muntjac can only be used directly on 70 cm and the QO-100 uplink. Clearly, with external transverters it could be used on any band.

When used with the Portsdown 4, the Muntjac will transmit QPSK with symbol rates of 125 kS to 1000 kS. It will also transmit 8PSK with symbol rates of 125 kS to 500 kS. It will not transmit at higher symbol rates because the RP2040 USB 1.1 interface is limited to a bus speed of 12 Mbit/sec.

The ouput power of the Muntjac is set by a gain parameter in the range 0 - 20. The shoulders on the output spectrum are always greater than 30 dB down, and are better than that at lower output powers. Approximate output power is shown in the table below.

Muntjac Performance

Gain Setting	437 MHz O/P	437 MHz Shoulders	2409 MHz O/P	2409 MHz Shoulders
20	+5.6 dBm	-34 dB	+4.1 dBm	-30 dB
19	+4.8 dBm		+3.7 dBm
18	+4.0 dBm		+2.8 dBm
17	+3.0 dBm		+2.1 dBm
16	+2.1 dBm		+1.2 dBm
15	+0.8 dBm	-40 dB	+0.5 dBm	-40 dB
14	+0.1 dBm		-0.5 dBm
13	-0.9 dBm		-1.4 dBm
12	-1.9 dBm		-2.5 dBm
11	-2.6 dBm		-3.3 dBm
10	-3.6 dBm	-50 dB	-4.4 dBm	-43 dB
9	-4.7 dBm		-5.3 dBm
8	-5.7 dBm		-6.3 dBm
7	-6.4 dBm		-7.3 dBm
6	-7.4 dBm		-8.3 dBm
5	-8.4 dBm	< -50 dB	-9.3 dBm	< -45 dB
4	-9.4 dBm		-10.3 dBm
3	-10.2 dBm		-11.1 dBm
2	-11.2 dBm		-12.1 dBm
1	-12.2 dBm		-13.1 dBm
0	-13.2 dBm	< -50 dB	-14.1 dBm	< -45 dB

The output power at 900 MHz is typically 3 dB less than that at 437 MHz.

Preparing for Use

The Muntjac is supplied as a kit with the final assembly step of soldering the output connectors to the PCB required for completion. If the SDR is to be mounted in a box (it does fit a small die-cast box) right-angle SMAs with long shafts should be used to allow sufficient clearance between the SMA plug and the box.

An example of the Muntjac mounted in a box (with short SMA Conectors) is shown here.

Using Muntjac with the Portsdown 4

The Muntjac connects to the Portsdown by USB. No other power supply is required.

After connecting the Muntjac to the Portsdown 4, select "Output to" and then "Muntjac". The first time you use each Muntjac device, it needs to be registered so that the Raspberry Pi recognises it at the next reboot. A message will pop up saying "System will reboot now to register new Muntjac. Touch screen to continue". Touch the screen, and after the reboot, your Muntjac is ready for use. You will not need to repeat this process unless you connect a different Muntjac, or you use a different Portsdown.

Next, set the Muntjac gain with reference to the table above.

Connect the correct output to your transmit amplifier; the Low output is used for 437 MHz (and 900 MHz) and the High output is used for 2400 MHz.

Within the limitations above, the Muntjac behaves similarly to a LimeSDR Mini.

IQ Calibration of the Muntjac

Poor IQ calibration in an SDR can cause a carrier to be transmitted at the centre frequency of a DATV transmission. There is a file in the Portsdown build that contains an average set of calibration parameters. Should IQ calibration for a particular Muntjac be a problem this file (/home/pi/rpidatv/bin/muntjac.mjo) will need to be amended.

Further guidance on how to determine the correct parameters and enter them in the file will be published in due course.

Programming the Muntjac

Muntjac SDRs are supplied with the correct program loaded in the RP2040. Should this need to be re-written, download this Zip file on a Windows PC and unzip it to get a .uf2 file File:muntjac4-0v1c.zip.

The current development release of Portsdown 4 (202601020) requires a later version of the RP2040 program: File:muntjac4_pico-1v0b.zip.

Press the BT (boot) button on the Muntjac whilst connecting it to the Windows PC by USB. A new disk drive labelled RPI-RP2 should appear on the PC; you can then release the button. Copy the .uf2 file into the RPI-RP2 drive. Once the file has copied, the disk drive will unmount. Disconnect the Muntjac from the Windows PC; it is ready for use with the Portsdown.