Chapter 25: Transceivers and Transverters

Lower cost, transmit and receive tuned together, simpler installation

Shared circuits cut the cost, one tuning control serves both, and a single box needs less space and fewer cables. The installation is simpler, not harder: it is the separate units that need the extra cabling.

Its receiver is more sensitive than a stand-alone one

Sharing circuits does not make the receiver more sensitive; the manual’s advantages are lower cost, tuning together and a smaller, simpler installation. Less space and fewer cables are part of that simpler installation.

Almost every one does, at least on its own transmit bands

Because of these advantages, nearly all modern amateur transmitters can also receive, at least on their own bands, and many add general-coverage receive. Separate units were the rule only in the early days.

Oscillators, filters, antenna switching, the microprocessor and the display

The manual lists oscillators or synthesisers, filters, antenna switching, the microprocessor and the display. One display shows the frequency and status on transmit and on receive alike, so it is shared too.

They can also receive outside the amateur bands

General coverage means the receiver also tunes the frequencies between and beyond the amateur bands, such as broadcast stations. The transmitter stays inside the amateur bands; sending outside them would break the licence conditions.

No signal it must receive can then fall on the IF itself

Inside the tuning range, signals on the IF could leak through the first mixer, and filtering them out would lose that frequency. It is not about noise figure; the high IF also moves images far off.

At 9 MHz, through the receiver’s 9 MHz filter

The filter method needs a narrow sideband filter, and the receiver’s 9 MHz IF filter is one. The 60 MHz roofing filter is about 12 kHz wide, far too wide to remove one sideband.

Its narrow SSB filter is at 9 MHz; the 60 MHz one is far wider

Chapter 24 makes the 60 MHz roofing filter about 12 kHz wide, to pass every mode, and puts the 2,4 kHz SSB filter at the 9 MHz IF. The roofing filter is far wider than SSB needs, not narrower.

About 2 to 3 kHz

It must pass speech of roughly 300 Hz to 3 kHz on one side of the carrier and stop the other sideband; chapter 24 quotes 2,4 kHz. A 12 to 15 kHz filter is a roofing filter.

The first LO, which is the synthesiser

Transmit runs the receive conversions in reverse, so the tuned first LO, the synthesiser, moves the transmit frequency just as it tunes the receiver. The second LO stays fixed, as it does on receive.

74,250 MHz

Transmit runs the receive conversion in reverse through the same synthesiser, so for the same frequency it stays put: 74,250 − 60 = 14,250 MHz. 45,750 MHz would mean retuning; 14,250 MHz gives no 60 MHz IF.

67,150 MHz

The synthesiser must sit the signal frequency away from the 60 MHz IF: 60 + 7,150 = 67,150 MHz, inside its range. 76,150 MHz adds the 9 MHz second IF too, but the synthesiser works against the first IF only.

Filters sharp enough for SSB work on one fixed frequency

Chapter 22 notes that filters selective enough to pass one sideband and reject the other are fixed-tuned, so SSB is made at one frequency and then mixed to the dial frequency. The balanced modulator itself could work anywhere.

RF in gives IF out, and IF in gives RF out

A passive diode mixer works either way round: an RF signal mixes with the LO to give the IF, and an IF signal mixes with it to give RF. The LO port only ever takes the LO.

The passive balanced diode mixer

Passive balanced mixers such as the diode ring work either way round, so one mixer can serve transmit and receive. A mixer with an amplifying transistor in it passes signals one way only.

Many filters pass signals just as well either way round

A filter of inductors, capacitors or crystals passes its band whichever end the signal enters, so many filters can serve both paths with no switching. That is how the crystal filter is shared.

VFO, crystal oscillator, mixer, product detector, crystal filter and one bandpass filter

Those six blocks carry the signal both ways, the product detector doubling as the balanced modulator. Amplifiers work one way only, so they come in switched pairs pointing in opposite directions.

The bandpass filter at the antenna input

On receive the antenna signal passes a bandpass filter, the RF amplifier and a second bandpass filter on its way to the mixer; only the second filter is shared. On transmit the antenna is fed through the lowpass filter.

Bandpass filter, RF amplifier, bandpass filter

The first bandpass filter and the RF amplifier are switched in for receive, and the second bandpass filter is shared with transmit. The lowpass filter belongs to the transmit path, after the PA.

Relays or solid-state switches

Several switches must change over together every time the operator transmits, so relays or solid-state (diode) switches do the job. Setting them by hand for every over would be slow and easy to get wrong.

Diode switches are faster and do not wear out

Chapter 15 credits PIN diode switches with no built-in lifetime limit and much faster switching than mechanical relays, and a microprocessor drives them easily. Higher power handling is not among the reasons given.

Too much gain would overload the mixer that follows

Chapter 24 warns against too much RF amplification ahead of the mixer, which strong signals would then overload; some designs leave the RF amplifier out. Gain changes signal levels, not where the image lies.

Passes only the wanted IF signal, rejecting all others

The mixer brings the wanted signal to the fixed IF, where the narrow crystal filter rejects everything else; this gives the receiver its selectivity. A filter has no gain: the amplifier after it does the amplifying.

3,900 MHz

With the VFO on 14,100 − 9 = 5,100 MHz, a 3,900 MHz signal also gives 3,900 + 5,100 = 9 MHz: twice the IF less the signal, 18 − 14,100. Adding twice the IF instead gives 32,100 MHz.

The unwanted sideband

The balanced modulator has already suppressed the carrier, leaving both sidebands; the crystal filter takes off the unwanted one, which is the filter method of making SSB. Harmonics are left to the lowpass filter.

Balanced modulator, making a double-sideband signal

Fed with the microphone audio and the crystal oscillator’s signal, the product detector becomes the balanced modulator, giving a double-sideband signal at the IF. The output frequency is set later, by the VFO and the other mixer.

The carrier for the balanced modulator

One fixed crystal oscillator serves both ways: it beats with the IF signal in the product detector on receive, and gives the carrier that the balanced modulator suppresses on transmit. The VFO does the mixing to the output frequency.

14,201 MHz

The tone becomes 9,000 + 0,001 = 9,001 MHz in the upper sideband, and adding the 5,200 MHz VFO gives 14,201 MHz. 14,200 MHz is the suppressed carrier, where the dial reads; the tone sits 1 kHz above it.

A VFO is easier to keep stable below 10 MHz

Chapter 22 advises keeping VFOs below about 10 MHz for stability, and 14 − 9 = 5 MHz meets that. A 23 MHz VFO would put the image on 23 + 9 = 32 MHz, well outside 20 m.

The unwanted mixing product

Mixing the IF signal with the VFO gives a sum and a difference; the bandpass filter passes the one on the transmit frequency and rejects the other. Harmonics are made later, by the PA, and the lowpass filter removes them.

Harmonics of the transmitted signal

A power amplifier makes harmonics at whole multiples of the output frequency, all above it, so a lowpass filter passes the signal and stops them. The unwanted mixing product was already removed by the bandpass filter after the mixer.

Its amplitude follows the speech, so distortion would spoil it

An SSB signal’s amplitude varies with the speech, so a non-linear PA would cause intermodulation distortion; chapter 22 suggests class AB. Class C is the more efficient, but it suits only constant-amplitude signals such as CW.

Mixer, bandpass filter, power amplifier, lowpass filter

The VFO’s mixer moves the SSB to the transmit frequency, the bandpass filter strips the other mixing product, the PA builds up the power and the lowpass filter removes its harmonics. Filtering before the PA avoids amplifying the unwanted product.

Unbalanced, 50 Ω

Most transceivers expect an unbalanced 50 Ω load, which suits the usual coaxial feed line. A balanced antenna or feeder needs a balun or matching unit between it and the transceiver’s coaxial socket.

A transmit converter and a receive converter in one box

A transverter pairs a receive converter with a transmit converter, normally in one enclosure, so that an existing transceiver can work another band. A transmitter and receiver sharing a VFO in one box is a transceiver.

To transmit and receive on a band it does not cover

A transverter converts the transceiver’s transmit signal up to the new band and received signals down to the transceiver’s band, so it covers another band both ways. Receiving alone would need only a receive converter.

22 MHz

With the LO below both bands, the 6 m frequency is drive plus LO: 28 + 22 = 50 and 30 + 22 = 52 MHz. 24 MHz pairs 28 with 52, putting 28-30 MHz on 52-54 MHz.

145,350 MHz, USB

The transmit mixer keeps the sum, 116 + 29,350 = 145,350 MHz, and a sum keeps the sideband the right way round: still USB. Only LO minus drive, like the rejected 86,650 MHz product, would invert it.

144,000 to 145,700 MHz

Each frequency moves up by the LO: 28,000 + 116 = 144,000 and 29,700 + 116 = 145,700 MHz. Reaching 146 MHz would need tuning up to 30 MHz, as the manual’s 28 to 30 MHz range assumes.

431,825 MHz

With the LO above the band, the output is the LO minus the drive: 460 − 28,175 = 431,825 MHz, and tuning runs backwards. Reading it as 432,175 MHz assumes the kHz digits carry straight across.

432,305 MHz

The output is the drive plus the LO, so any LO error is added straight on: 144,300 + 288,005 = 432,305 MHz. Ignoring the error gives 432,300 MHz; the transceiver’s accurate dial cannot correct the LO.

Offset it to show the real frequency on the new band

With the offset set, the display adds the LO for the operator, so tuning 28 to 30 MHz reads as the real frequency on the new band. Showing the fixed LO would read the same on every channel.

About 0 dBm (1 mW)

Most transverters expect about 0 dBm, 1 mW, and have their own power amplifier; −30 dBm is a thousand times less. Some newer units take a few watts through a built-in attenuator, but 100 W would ruin most.

44 dB

25 W is 25 000 mW, and 10 log 25 000 = 44 dB, so a 44 dB attenuator leaves 1 mW. 14 dB is 25 W in dBW, measured from 1 W instead of 1 mW.

45 dB

35 W is 35 000 mW, and 10 log 35 000 = 45,4 dB: nearest 45 dB. Measuring from 1 W instead gives 10 log 35 = 15,4 dB, which would leave about 1 W, a thousand times too much.

10 mW

38 dB is a power ratio of about 6 310, so 70 W ÷ 6 310 = 11 mW: nearest 10 mW, ten times the usual 0 dBm. Treating 38 dB as a voltage ratio, 79, leaves about 0,9 W.

40 dB

12 W is 12 000 mW, and 10 log 12 000 = 40,8 dB: nearest 40 dB. 11 dB is 12 W in dBW, a ratio taken from 1 W instead of from the 1 mW drive.

Most transverters would be damaged by its full output

A typical HF transceiver gives 100 W, while most transverters expect about 1 mW and would be damaged by full power. The attenuator simply throws the excess away; blocking LO leakage is not its purpose.

A low-level RF drive signal

A transverter socket delivers a low-level RF drive signal, so no attenuator is needed. Without one, an attenuator must cut the transceiver’s normal output down to what the transverter can safely take.

5 mW

+7 dBm is 7 dB above 1 mW, and 7 dB is a power ratio of 5, so 5 mW. 2,2 mW applies the 20 log voltage rule to power; 7 mW and 5 W misread dBm.

The transceiver, through its T/R control output

The transceiver’s T/R control output changes the transverter over at the same moment as the transceiver itself, so the PA and the receive preamplifier are in the right path. Waiting to sense the RF drive is slower and less certain.

Change to transmit at once, prompted by the T/R control output

Through the T/R control line the transverter changes over in step with the transceiver, so its PA is in line before drive arrives. Waiting for the drive would send the first of it into the receive side.

The power amplifier and the low-noise preamplifier

The LO, the mixer and all three bandpass filters serve both directions, as in a transceiver; only the amplifiers, which work one way, are switched: the PA for transmit, the low-noise preamplifier for receive.

Lifting weak signals and making up for the mixer’s loss

The low-noise preamplifier lifts weak 2 m signals and makes up for the loss in the mixer that follows. Matching the antenna is a job for the antenna system and feed line, not the preamplifier.

6 to 7 dB

Chapter 20 puts a diode mixer’s conversion loss at 6 to 7 dB: each output product is that much weaker than the RF input. Being passive, a diode mixer cannot give gain.

12 dB

Losses count as negative gains: 20 − 6,5 − 1,1 = 12,4 dB, nearest 12 dB. Leaving out the filters gives 13,5 dB, and adding the losses as if they were gains gives about 28 dB.

At VHF the receiver’s own noise, not band noise, sets the limit

Chapter 23 notes that at HF the band noise swamps the receiver’s noise, but at VHF and above the receiver’s own noise becomes the limit, hence low-noise preamplifiers. Band noise far above receiver noise is the HF case.

The unwanted difference product

The mixer gives the 2 m sum and a difference product at 86-88 MHz, below the LO; this filter keeps the sum and rejects the difference. Harmonics arise later, in the PA, and the final filter removes them.

Harmonics of the output signal

The power amplifier adds harmonics at multiples of the 2 m output, and the final bandpass filter, centred on 144-146 MHz, stops them. The difference product was already rejected by the filter straight after the mixer.

Its sideband is inverted: USB comes out as LSB

As the incoming frequency rises, LO minus signal falls, so the spectrum is mirrored and USB becomes LSB, like the inverted difference product in the manual’s transverter. A sum, or signal minus LO, keeps the sideband.

LSB on 29,800 MHz

The output is the LO minus the drive: 80 − 29,800 = 50,200 MHz. Taking the drive from the LO mirrors the spectrum, so the transceiver must send LSB for the 6 m signal to come out as USB.

It falls from 52,000 to 51,500 MHz

The 6 m frequency is 80 MHz minus the transceiver’s: 80 − 28,000 = 52,000 and 80 − 28,500 = 51,500 MHz, so tuning up moves it down. An LO below the signal would keep both moving the same way.

Signals on the image frequencies

Ahead of the mixer, the filter keeps out image signals on the far side of the LO, which would also convert to 28-30 MHz. The sum product appears only after the mixer, where the 28-30 MHz filter removes it.

The 28-30 MHz bandpass filter on the transceiver side

The sum, 144-146 + 116 = 260-262 MHz (p222’s 262-264 MHz is a slip), appears only after the mixer, so only the 28-30 MHz filter can stop it; the 144-146 MHz filters come before the mixer.

260,825 MHz

The mixer gives the difference, 144,825 − 116 = 28,825 MHz, the wanted IF, and the sum, 144,825 + 116 = 260,825 MHz, which the 28-30 MHz filter rejects. 87,175 MHz is the image frequency, an input, not an output.

It reaches 28-30 MHz with the signal, inside the transverter

An image near 86-88 MHz mixes with the 116 MHz LO to land in 28-30 MHz among the wanted signals, so the transceiver cannot tell them apart. It must be filtered out before the transverter’s mixer.

86,750 MHz

The IF is 145,250 − 116 = 29,250 MHz; the image lies that far below the LO, at 86,750 MHz (p222’s 88-90 MHz is a slip). 203,750 MHz adds twice the IF, as for an LO above.

Its image then lies some 56 MHz away, easy to filter out

The image lies twice the IF from the signal: about 56 MHz off with a 28 MHz IF, only 7 MHz with 3,5 MHz. Band noise on 10 m is irrelevant: the IF comes by cable.

The transceiver’s own IF filters

The transverter only shifts the band: its filters are a whole band wide, 2 MHz. Signals then pass through the transceiver as through any superhet, so its narrow crystal filter decides how close a neighbour can be.

A variable-frequency first IF

The converter’s fixed LO shifts the whole new band down, and the receiver tunes across the shifted range, so its front end acts as a tunable first IF. A fixed first IF would need a tunable LO in the converter.

A triple-conversion superhet

The converter adds one more frequency conversion in front: the receiver’s tuning range becomes a variable first IF, and its two IFs become the second and third. Two conversions plus one make three.

3 conversions

Each converter adds one conversion ahead of the receiver: 23 cm to 2 m, 2 m to HF, then the superhet’s own mixer to its IF. Leaving out one of the converters gives two.