I and Q Phasing Modulators


I & Q Modulator for HF

Using the fourth Method


Transmitting DRM is quite strait ford by contracting an I & Q modulator for HF (80 meter band). The DRM dream has a software encoder that can be set up for an I & Q output. DRM dram uses both in and out on your PC sound card.

The I & Q modulator is made up of logic ICs and a set of op-amps. The input signal must be four times above the wanted transmit frequency. 74HC04 hex inverter is configured as high gain signal amplifier with a 10M feed back resistor. 74HC74 and the 74HC08 are setup as a Johnson ring counter. The 74HC4066 is the high-speed switch, which is driven by the Johnson ring counter. TL074 op-amp lets you setup the I & Q input levels and provides a 180 phase shift network. NE5534 is a 10 MHz gain bandwidth op-amp, that is a buffer to provide a high input impedance and to drive a low output impedance. The BC549 and the BC559 transistors are a low-level RF amplifier stage to feed in to 50 ohms. Output filter is there to remove unwanted higher frequency components within the transmitted RF signal.

Phasing modulator circuit full size

One watt HF amplifier

The output form the I & Q modulator is feed into this one watt amplifier. The BC549 is a small signal high gain amplifier stage. 2N3053 is DC cupped from the BC549 stage and provides a low impedance drive stage. The two 2N4427 transistors are parallel in the output stage which provides a match into 50 ohm.



HF Phasing transceiver

My HF transceiver uses a DDS for the local oscillator. It operates a four times the RF frequency that you are on. My frequency range is from near DC to just over 20 MHz. The transceiver has been design in a way to take, both the I and Q signals in and out. To allow a computer sound card to connected.

The photo on the left is of my transceiver

On the top is the SSB generator and below is the I & Q RX. / TX

Software that I have used with this transceiver, are:

Winrad, DRM Dream and Spark DRM Transmitter.

DRM Modulation

Also used RFSM-2400 for file transfers (not I & Q compatible).

Winrad by Alberto I2PHD Link

I and Q Phasing

The differences between these two signals I & Q is 90 at all frequencies. There are two ways you can look this, the first is a base-band signal and second is an IF signal. My HF transceiver uses the base-band approach. For SSB one of the inputs is shifted by 90 degrees compared to the other. Over a frequency range of 300 Hz to 3 kHz, this is done with in a polyphase network.

Most software programs take an IF approach. The IF frequency is normally adjustable around the centre frequency of 12 kHz. The IF frequency range is fix by the PC sound card sampling frequency.

The modulator works by using two double balance mixers off set by 90. The outputs are then combined into one output. The unwanted side band is cancelled out by its phase. For example:

I in is 0 + 0 = 0 where Q in is 90 + 90 = 180

I becomes in phase and Q is out of phase.

By swapping over I & Q to Q & I you change from upper side band to lower side band for SSB. Will also change the IF from the hi side, to the lower side of the carrier frequency.

The two waveforms displayed on my oscilloscope, show the I & Q signals at 1 kHz

The Tayloe mixer

Here is a block diagram of the 4th method. Shows the internal workings of my I and Q transceiver. This method uses all four-phase angles unlike the 3rd method. With this type of mixer there is amount of fourth harmonic brake through. That is why there is an output filter used.

The diagram shows my microphone base-band filter for my SSB generator. That needs to filtered before the signal feed on to the phase shift network.

The block diagram shows my SSB demodulator/modulator. Uses a single Polyphase network for RX and TX that is switch.

On the receive side there is adjustable 500 Hz to 5 kHz notch filter. The AGC is a NE572 compandor chip. Where the rectified DC is amplified by an op-amp, and feed back to the receiver front end pre-amp. After the compandor the line level is spit two ways. One to the DSP board the other comes out as line out. The DSP unit works as a noise eliminating processor setable across four levels of processing. Good removing QRN from the 80 meter band. Next is on to the 4-watt audio amplifier stage.

On the TX side electric-microphone is used. That is feed on to an audio pass band filter that you can see displayed above. After the filter line in is combined at this point. And again the TX signal is switch through a common Polyphase network.

Below is a diagram of a polyphase network that I have used.