IQ Upconverters for SDR Transmitters
For use with the G3PLX SDR Transmitter Driver.

The three designs here, which have all been tested and bread boarded, will perform the single-sideband upconversion of the stereo I/Q audio output from a PCs soundcard to a low level of RF suitable for driving a transmitter. The Audio I/Q inputs can be generated from microphone input by using the
SDR Transmitter Driver software written by G3PLX, or from other specific data-mode software that is designed with an I/Q output option. At the moment, only a few software packages provide this, but it is relatively straightforward to implement, so hopefully those software writers who have not provided the option, will soon do so.

High Spec HF Upconverter design.

This design can almost be taken as the Rolls-Royce of I/Q upconverter designs, and provides, probably, one of the best combinations of carrier null and sideband suppression likely to be seen in amateur practice. It uses a pair of D-Type flip-flops constructed from high speed logic devices to divide the input reference by four and generate the precise 0/90 degree Local Oscillator drive to the mixers. These are made from a rather old, but still very effective, four-quadrant multiplier chip based around the Gilbert Cell , or transistor tree, approach. The MC1496 chip offers full opportunity for carrier nulling, and this, taken with individual amplitude trimming allows a carrier rejection in excess of 40dB to be achieved. In practice, over a limited frequency range, a value exceeding 55dB has been seen.

The circuit is shown in Figure 1 , and although it appears to be rather complex, much of the circuitry consists of bias and carrier nulling resistors. Again, an opamp buffer is included on the audio input to offer a high input impedance and drive the mixer.

The Local oscillator drive has to be at four times the required output frequency, the logic family used here will function satisfactorily to beyond 80MHz, so this upconverter can generate RF at up to 20MHz.

The output from the two devices is ideally combined using a ferrite transformer using a bifilar-wound, centre tapped primary to achieve accurate balance. Minicircuits do a range of suitable transformers. Alternatively, the outputs of the MC1496 devices could just be connected in parallel for reduced output level capability.

To set up, apply a pair of I/Q audio tones to the I/Q inputs and a four-times LO drive of around 0dBm to the reference input. If there is any opportunity for software balance of the I/Q tones in the driving software, ensure this is disabled or the values set to zero. Monitor the output on a receiver, and ensure one sideband at the tone frequency plus (or minus) the carrier is at a noticeably lower level than the other. Also, check the level of LO leakage which should also be considerably lower than the wanted sideband. Swap over the audio drive polarity, either by swapping cables, or in software, and ensure the two sidebands swap over.

Adjust the carrier null on each chip to minimise the carrier leakage. This can conveniently be done one at a time with just one audio channel applied. A carrier null of at least 40dB below maximum RF level of the wanted sideband should be achievable on each side of the upconverter. Re-apply both audio inputs and adjust the relative gain to minimise the level of the unwanted sideband. There is no opportunity for phase trimming on this board as the digital I/Q generator is near-perfect at frequencies up to 40MHz (10MHz output) and only slightly worsening at it approaches its maximum frequency capability. An output level of up to around +5dBm can be achieved before the devices start to saturate, with potential loss of cancellation performance, and of linearity.

144 MHz Upconverter.

This uses a completely different approach to generating the 90 degree RF phase shift. Figure 2 shows how a Minicircuits PSCQ-2-160 quadrature hybrid can be used to generate 0/90 degree drive to a pair of diode ring mixers. This device is specified to operate over the frequency range of 100 to 160MHz. and the specification sheet available from Minicircuits specifies a phase error of less than 1 degree at 145 MHz, and amplitude error of less than 0.7dB. In practice, the results observed on several of these devices have been quite a lot better than this. According to Minicircuits data the PSCQ-2-180 should perform even better at 145MHz, but this device has not been tested.

A MAR-3 modamp amplifies the LO drive up to a level of 13dBm, which is split within the hybrid to give 10dBm to each mixer. Opamp buffers provide a low impedance drive for the mixers. The mixer drive at audio passes through a potential divider made from a pair of 100 ohm resistors. These serve the dual purpose of ensuring the mixer IF port sees a 50 ohm drive impedance which is necessary for correct linear mixer operation, and also increases the impedance the opamp has to drive to around 125 ohms. The outputs from the two mixers are combined in a resistive combiner and amplified up to around +6dBm by a further modamp.

No amplitude or phase trimming has been provided in the hardware, but both could be added if necessary. An extra 3dB output level could be achieved, as well as a potential improvement in nulling, if the resistive combiner is replaced by a hybrid combiner such as the Minicircuits PSCJ-2-1

Simple HF Upconverter

A simpler HF design that that above can be made using NE612 mixer chips- the circuit is shown in Figure 3. These eight pin chips give no opportunity for carrier nulling, but require far fewer components around them to get going; they also operate at a significantly lower power level and a maximum output of no more than 0dBm should be expected. However, they can be pressed into service at higher frequencies, although the quadrature LO generation circuitry will then have to be replaced by something more suitable.

Other Design Ideas

There are many custom made I/Q upconverter chips on the market now, as I/Q conversion is central to much consumer communication equipment such as mobile phones. They typically work over a frequency range that often encompasses a (microwave) amateur band or two. These devices are often short-lived, being manufactured in their millions for a production run then abandoned to be replaced by a better (or cheaper) version. Such chips often become available on the surplus market or new from the suppliers, and provided you can get the data sheet, will work well with the SDRTX software. If purchased in bulk they can be very cheap. See the range of products from RF Microdevices , for example. or the AD8346 from Analog Devices covering 800MHz to 2.7GHz

A range of 90 degree phase shift networks can be used with separate mixers . A favourite using lumped elements is the high/low pass Pi network that can give up to an octave frequency coverage. At microwave frequencies, an additional quarter wave of transmission line in one leg can provide a suitable 90 degree phase shift for two mixers over a narrow band.