TX Mixer (QSE) Introduction

General

This stage adds the TX Mixer to the board and provides the modulation of the Dividers' output signals by the 4 I and Q signals from the TX OpAmps (Quadrature Sampling Exciter - QSE). The result is a double sideband RF waveform that will be coupled into the PA Stage.

Theory of Operation

Let's suppose you were working 40m with a SDR center frequency of 7.056 MHz. You have tuned your SDR (via the software) so that you are listening to a CW station on 7.066 MHz (10 kHz up from the center frequency).

If you were to listen to the I and Q signals from the hardware RX via an amplifier or headphones (assuming there were no other signals and little noise) you would hear a single 10 kHz CW signal on both channels. Your ears may or may not be able to detect the fact that the tone in one ear is 90 degrees out of phase (I.e., In quadrature) with the tone in the other ear.

By comparing the phase difference between the I/Q signals the SDR RX software determines that this signal is 10KHz higher than the center frequency and displays the CW signal as a broken line on the waterfall. It also frequency shifts this signal at the output of the soundcard so it is heard at as normal CW tone from the speakers.

Now, assume you decide to transmit on that frequency where you have placed your cursor (7.066 MHz). Your SDR software will emit a 10 kHz signal (the delta between the center frequency and the selected TX frequency) in both the I and Q outputs, again, in quadrature. This will go into the SDR hardware on the TX side, through the TX OpAmps and into the TX Mixer, where it will beat up against the LO center frequency of 7.056 MHz. The input to the mixer is 10KHz from the soundcard; the output from the mixer is the RF product of the LO and the input 10 kHz signal. If you were to listen to the input 10 kHz signal you would hear a tone. This tone is audible simply because it represents the delta between the center frequency and the desired transmit frequency, in this case, only 10 kHz, and, since it is in the range of audio frequencies, is audible. If you were to select a higher frequency on whch to transmit, say 7.080 MHz, the resultant signal would be a 24 kHz signal; perhaps your dog would hear it, but you certainly would not! However, the TX Mixer will happily upconvert this to an RF product!

The TX OpAmps are unity gain and serve to split the incoming I and Q signals into 4 components: 0, 180, 90, and 270 degree phase. Each of the four are then input to the TX Mixer and are switched ("mixed" by the LO signals which are 90 degrees apart in phase). The two outputs of the mixer are the up-converted RF products, in anti-phase (I.e., 180 degrees apart). These RF products are fed to the Driver/PA Stage. Via a transformer (T2) that will cancel out one or the other of the anti-phased RF signals out, depending upon which is leading and which is lagging. This is just like the RX, any minor phase errors are compensated. The software will make the phase and level of the I/Q signals such that the mixing product 10KHz below 7.056 is cancelled leaving a signal 10KHz above 7.056 at the output of the TX Mixer'sT2..

The SDR software sends two suitably phased signals from the soundcard it is not that tone that is transmitted, but an up-converted mixer RF product, amplified and filtered in the PAF board.

Note: the above description relates to the traditional "Softrock/Rocky/Winrad/Genesis fixed center frequency" paradigm. In the PowerSDR paradigm, things are a little different. On receive, all versions of PSDR (other than the Genesis version) make use of a pseudo IF system, nominally set at 9kHz. When you go to transmit, the LO frequency is shifted by 9kHz so that it is actually at the wanted TX frequency. That has the distinct advantage that the TX image rejection is always carried out a the same sound card frequency range, i.e. DC to say 3 or 4kHz.

For Rocky and the Genesis PSDR version, both use a 'fixed' LO for their centre frequency, so when you transmit, the sound card has to generate an output at whatever frequency offset you are 'tuned' to, so if you were 30kHz below the centre frequency, then the soundcard I/Q output would be 30kHz and the TX 'audio' signal. Doing it that way means the TX image rejection is always compromised as it cannot be optimised at a single point, as you could be transmitting anywhere +/- the sound card sampling rate.

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TX Mixer (QSE) Schematic

(Resistor testpoints (hairpin, top, or left-hand lead), as physically installed on the board, are marked in the schematic with red dots)

(Click for Full Schematic)

(above schematic has clickable areas that can be used for navigation)

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TX Mixer (QSE) Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

Check	Count	Component	Marking	Category
❏	1	2.21 k 1/4W 1%	r-r-br-br-br	1/4W
❏	1	22.1 k 1/4W 1%	r-r-brn-r-br	1/4W
❏	1	3.32 k 1/4W 1%	ora-ora-red-brn-brn	1/4W
❏	1	4.99 k 1/4W 1%	y-w-w-br-br	1/4W
❏	2	49.9 ohm 1%	yel-wht-wht-gld-brn	1/4W
❏	1	4.7 uF 10% 16V X7R RAD	475	Ceramic
❏	1	band-specific		misc
❏	6	band-specific		misc
❏	2	0.1 uF	(smt) black stripe	SMT 1206
❏	1	FST3253 mux/demux switch	FST3253	SOIC-16
❏	1	2N3904 NPN Transistor	2N3904	TO-92

Band Specific Items for 160m Band

Check	Designation	Component	Marking	Category	Orientation	Notes	Circuit
❏	C20	1000 pF 5%	102	Ceramic			TX Mixer (QSE)
❏	C21	270 pF 5%	271	Ceramic			TX Mixer (QSE)
❏	C64	0.1 uF	(smt) black stripe	SMT 1206			TX Mixer (QSE)
❏	L1	30 uH 83T #30 on T30-2 (red) (44")	red	Coil			TX Mixer (QSE)
❏	L1-core	T30-2 toroid core	red	Toroid			TX Mixer (QSE)
❏	T2	7.1 uH 20T bifilar/40T #30 on T30-2 (red) (22")	red	Xfrmr			TX Mixer (QSE)
❏	T2-core	T30-2 toroid core	red	Toroid			TX Mixer (QSE)

Band Specific Items for 80, 40m Band

Check	Designation	Component	Marking	Category	Orientation	Notes	Circuit
❏	C20	220 pF 5%	221	Ceramic			TX Mixer (QSE)
❏	C21	220 pF 5%	221	Ceramic			TX Mixer (QSE)
❏	C64	0.1 uF	(smt) black stripe	SMT 1206			TX Mixer (QSE)
❏	L1	4.7 uH 33T #30 on T30-2 (red) ("19")	red	Coil			TX Mixer (QSE)
❏	L1-core	T30-2 toroid core	red	Toroid			TX Mixer (QSE)
❏	T2	5.0 uH 17T bidfilar/34T #30 on T30-2 (red) (19")	red	Xfrmr			TX Mixer (QSE)
❏	T2-core	T30-2 toroid core	red	Toroid			TX Mixer (QSE)

Band Specific Items for 40, 30, 20m Band

Check	Designation	Component	Marking	Category	Orientation	Notes	Circuit
❏	C20	100 pF 5%	101	Ceramic			TX Mixer (QSE)
❏	C21	100 pF 5%	101	Ceramic			TX Mixer (QSE)
❏	C64	0.1 uF	(smt) black stripe	SMT 1206			TX Mixer (QSE)
❏	L1	2.3 uH 25T #30 on T30-6 (15")	yellow	Coil			TX Mixer (QSE)
❏	L1-core	T30-6 toroid core	yellow	Toroid			TX Mixer (QSE)
❏	T2	2.43 uH 13T(bi)/26T #30 on T30-6 (18")	yellow	Xfrmr			TX Mixer (QSE)
❏	T2-core	T30-6 toroid core	yellow	Toroid			TX Mixer (QSE)

Band Specific Items for 30, 20, 17m Band

Check	Designation	Component	Marking	Category	Orientation	Notes	Circuit
❏	C20	82 pF	82J	Ceramic			TX Mixer (QSE)
❏	C21	82 pF	82J	Ceramic			TX Mixer (QSE)
❏	C64	0.1 uF	(smt) black stripe	SMT 1206			TX Mixer (QSE)
❏	L1	1.6 uH 21T #30 on T30-6 (yellow) (14")	yellow	Coil			TX Mixer (QSE)
❏	L1-core	T30-6 toroid core	yellow	Toroid			TX Mixer (QSE)
❏	T2	1.74 uH 11T bifilar/22T #30 on T30-6(yellow) (14")	yellow	Xfrmr			TX Mixer (QSE)
❏	T2-core	T30-6 toroid core	yellow	Toroid			TX Mixer (QSE)

Band Specific Items for 15, 12, 10m Band

Check	Designation	Component	Marking	Category	Orientation	Notes	Circuit
❏	C20	47 pF 5%	47J	Ceramic			TX Mixer (QSE)
❏	C21	22 pF 5%	22J	Ceramic			TX Mixer (QSE)
❏	C64	0.01 uF	(smt)	SMT 1206			TX Mixer (QSE)
❏	L1	2.1 uH 24T #30 on T30-6 (yellow) (15")	yellow	Coil			TX Mixer (QSE)
❏	L1-core	T30-6 toroid core	yellow	Toroid			TX Mixer (QSE)
❏	T2	0.81 uH 8T bifilar/15T #30 on T30-6 (yellow) (11")	yellow	Xfrmr		2223 and 2225	TX Mixer (QSE)
❏	T2-core	T30-6 toroid core	yellow	Toroid			TX Mixer (QSE)

TX Mixer (QSE) Summary Build Notes

• Wind and Install Inductors
• Install Topside Components
• Install Bottomside Components
• Test the Stage

TX Mixer (QSE) Detailed Build Notes

Top of the Board

Bottom of the Board

TX Mixer (QSE) Completed Stage

Top of the Board

Bottom of the Board

TX Mixer (QSE) Testing

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