RX Mixers/OpAmps Introduction


The G3020's mixer is a double-balanced, sample and hold circuit using two quad bilateral analog switches (74HC4066), clocked by the four outputs from the dividers.

Each pair of switches outputs to an RC pair that samples and holds the output for forwarding to the OpAmps Section. The portion of the schematic illustrating this is shown below (including divider output waveforms derived from a simulation of the divider operation). Not shown is that part of the circuit that uses two 5k trimpots for hardware balancing of the mixer.

The incoming RF is split into 2 streams, each going to two pairs of switches. Each switch pair is turned on and off by a signal coming from the dividers. The switched outputs of the pair are integrated by the corresponding RC circuit. As in the Tayloe detector, the result is the down-conversion of the RF signals to their AF analogues.

In the OpAmps sub-stage, the down-converted audio frequency signals (in quadrature) from the mixers are combined (0° + 180°, 90° + 270 °) and then summed and passed to the PC, either directly following the summing operation or passed through an amplification stage for a 23 db boost (as determined by S1), as I and Q outputs of the G3020.

The sub-schematic below shows the RX path for the opamps. (A similar path is defined for the TX opamps, where the I and Q signals from the PC are sent to the mixers and out to the RF line.)

(go directly to build notes)

RX Mixers/OpAmps Schematic

RX Mixers/OpAmpsschematic

(go directly to build notes)

RX Mixers/OpAmps Bill of Materials

Stage Bill of Materials

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

101 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W
4100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W
1210K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W
1150 1/4W 1%brn-grn-blk-blk-brn brn-grn-blk-blk-brn1/4W
533 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W
1330 1/4W 1%ora-ora-blk-blk-brn ora-ora-blk-blk-brn1/4W
23k3 1/4W 1% (3.3K)ora-ora-blk-brn-brn ora-ora-blk-brn-brn1/4W
247K 1/4W 1%yel-vio-blk-red-brn yel-vio-blk-red-brn1/4W
2JRC27F/012 relayJRC27F/012 JRC27F/01212V relay
21N41481N4148 1N4148Axial
110 nF (.01uF)103 103Ceramic
14100 nF104 104Ceramic
247 uH molded inductor 10%yel-vio-blk-slv yel-vio-blk-slvChoke
1shunt wire (cut-off lead)Cutoff
274HC4066 Quad Bilateral SW DIP-14
4NE5532 Dual OpAmpNE5532 NE5532DIP-8
110 uF/25Vdc Electrolytic
12100 uF/25Vdc Electrolytic
2220 uF/25Vdc Electrolytic
1misc hookup wireHookup
1BNC Connector Male - PCB mount Jack
13.5mm stereo jack - PCB mount (rt-angle) Jack-RA
2100 nF USM104J 104JMetalized polyfilm
422 nF USMF98 223J or F87 223J F98 223J or F87 223JMetalized polyfilm
410 uF (NP) Non-Polarized
1500 (Y501)Y501 Y501Trimpot
25K (Y502)Y502 Y502Trimpot

RX Mixers/OpAmps Summary Build Notes

RX Mixers/OpAmps Detailed Build Notes

Top of the Board

RX Mixers/OpAmps Top View

Install Diodes and Resistors

Install the resistors and the diodes.

Install Diodes and Resistors photo
D5-11N41481N4148 1N4148Axial 
D5-21N41481N4148 1N4148Axial 
R5-133 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R5-233 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R5-333 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R5-433 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R5-533 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R5-6100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
R5-7100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
R5-8100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
R5-9100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
R5-10150 1/4W 1%brn-grn-blk-blk-brn brn-grn-blk-blk-brn1/4W 
R5-11330 1/4W 1%ora-ora-blk-blk-brn ora-ora-blk-blk-brn1/4W 
R5-121 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-131 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-141 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-151 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-161 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-171 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-181 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-191 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-201 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-211 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R5-223k3 1/4W 1% (3.3K)ora-ora-blk-brn-brn ora-ora-blk-brn-brn1/4W 
R5-233k3 1/4W 1% (3.3K)ora-ora-blk-brn-brn ora-ora-blk-brn-brn1/4W 
R5-2410K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-2510K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-2610K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-2710K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-2810K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-2910K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3010K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3110K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3210K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3310K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3410K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3510K 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5-3647K 1/4W 1%yel-vio-blk-red-brn yel-vio-blk-red-brn1/4W 
R5-3747K 1/4W 1%yel-vio-blk-red-brn yel-vio-blk-red-brn1/4W 

Install NPO and USM Capacitors

Install the ceramic capacitors (and the two "boundary" 100 uF electrolytics).

Note that there are two types of ceramic capacitors here: the normal 103s and 104s and the larger "USM" capacitors which are colored differently.

Install NPO and USM Capacitors photo
C5-122 nF USMF98 223J or F87 223J F98 223J or F87 223JMetalized polyfilm 
annotated "USM" on the graphic
C5-222 nF USMF98 223J or F87 223J F98 223J or F87 223JMetalized polyfilm 
annotated "USM" on the graphic
C5-322 nF USMF98 223J or F87 223J F98 223J or F87 223JMetalized polyfilm 
annotated "USM" on the graphic
C5-422 nF USMF98 223J or F87 223J F98 223J or F87 223JMetalized polyfilm 
annotated "USM" on the graphic
C5-9910 nF (.01uF)103 103Ceramic 
C5-10100 nF104 104Ceramic 
C5-11100 nF104 104Ceramic 
C5-12100 nF104 104Ceramic 
C5-13100 nF104 104Ceramic 
C5-14100 nF104 104Ceramic 
C5-15100 nF104 104Ceramic 
C5-16100 nF104 104Ceramic 
C5-17100 nF104 104Ceramic 
C5-18100 nF104 104Ceramic 
C5-19100 nF104 104Ceramic 
C5-20100 nF104 104Ceramic 
C5-5100 nF USM104J 104JMetalized polyfilm 
annotated "USM" on the graphic
C5-6100 nF USM104J 104JMetalized polyfilm 
annotated "USM" on the graphic
C5-7100 nF104 104Ceramic 
C5-8100 nF104 104Ceramic 
C5-9100 nF104 104Ceramic 
C5-39100 uF/25Vdc Electrolytic 
C5-40100 uF/25Vdc Electrolytic 

Install Ics

Install the 4 OpAmp Ics (using ESD precautions and taking care to orient them correctly - see the graphic above). The Ics are oriented so that their "1" pins are in the upper right, when viewing the board with the Power supply at the top.

Install Ics photo
U5-174HC4066 Quad Bilateral SW DIP-14 
Take ESD precautions
U5-274HC4066 Quad Bilateral SW DIP-14 
Take ESD precautions
U5-3NE5532 Dual OpAmpNE5532 NE5532DIP-8Pin 1 is to upper right
Take ESD precautions
U5-4NE5532 Dual OpAmpNE5532 NE5532DIP-8Pin 1 is to upper right
Take ESD precautions
U5-5NE5532 Dual OpAmpNE5532 NE5532DIP-8Pin 1 is to upper right
Take ESD precautions
U5-6NE5532 Dual OpAmpNE5532 NE5532DIP-8Pin 1 is to upper right
Take ESD precautions

Install Remaining Components

Prior to installing the Y502 trimpots, check their resistance; each leg should have 2.5kOhms - i.e., the trimpots should be set exactly in the middle of the 5kOhm range. This should be the factory setting, but it does not hurt to check just in case these may have been jostled in shipment/handling. These trimpots will later be used for TX carrier suppression adjustments, if any. They are will be adjusted in this phase (and in the Final Tests and Adjustments page).

Note that there are 4 "can-like" capacitors that can easily be confused as polarized electrolytic capacitors. However, there 4 10 uF capacitors are NON-polarized ("NP"), with no need to be concerned about + and minus otientations. There is another 10uF capacitor that is polarized. Do not get the 2 types confused! All of the polarized electrolytics have that grey stripe with the minus sign down the side; the "NP" capacitors do not have that stripe. Refer to the BOM for visuals.

Install Remaining Components photo
R5-38500 (Y501)Y501 Y501Trimpot 
R5-395K (Y502)Y502 Y502Trimpot 
Needs to be initialized (see notes above)
R5-405K (Y502)Y502 Y502Trimpot 
Needs to be initialized (see notes above)
L5-147 uH molded inductor 10%yel-vio-blk-slv yel-vio-blk-slvChoke 
L5-247 uH molded inductor 10%yel-vio-blk-slv yel-vio-blk-slvChoke 
C5-3510 uF (NP) Non-Polarized 
Don't confuse with 10 uF electrolytic
C5-3610 uF (NP) Non-Polarized 
Don't confuse with 10 uF electrolytic
C5-3710 uF (NP) Non-Polarized 
Don't confuse with 10 uF electrolytic
C5-3810 uF (NP) Non-Polarized 
Don't confuse with 10 uF electrolytic
K5-1JRC27F/012 relayJRC27F/012 JRC27F/01212V relay 
K5-2JRC27F/012 relayJRC27F/012 JRC27F/01212V relay 
C5-2210 uF/25Vdc Electrolytic 
Don't confuse with 10 uF non-polarized ("NP")
C5-23100 uF/25Vdc Electrolytic 
C5-24100 uF/25Vdc Electrolytic 
C5-25100 uF/25Vdc Electrolytic 
C5-26100 uF/25Vdc Electrolytic 
C5-27100 uF/25Vdc Electrolytic 
C5-28100 uF/25Vdc Electrolytic 
C5-29100 uF/25Vdc Electrolytic 
C5-30100 uF/25Vdc Electrolytic 
C5-31100 uF/25Vdc Electrolytic 
C5-32100 uF/25Vdc Electrolytic 
C5-33220 uF/25Vdc Electrolytic 
C5-34220 uF/25Vdc Electrolytic 

Install Connectors

Install Connectors photo
P5-13.5mm stereo jack - PCB mount (rt-angle) Jack-RA 
P5-2BNC Connector Male - PCB mount Jack 
Careful: there are two places for BNC. This one goes in the lower location

Temporarily Connect Antenna And Mixer Input

Tack-solder a wire to connect the antenna "hot" pin on the BNC connector to the Mixer Stage's Input point on the board, as indicated in the graphic.

Builder may choose to install this temporary wire from the bottom or from the top:

(This wire will be removed in the next phase.)

Installing from Topside
Topside installation of temporary wire
Installing from Bottomside

Temporarily Connect Antenna And Mixer Input photo
TMP5-1misc hookup wireHookup 
temporary connection

Temporarily enable IQ Output

Temporarily connect the pins for switches S1A and S1B, to enable I and Q output to the ("To Sound Card") stereo jack. This involves shorting together pins 3 and 2 in each of the two sets of holes flanking the jack (i.e., simulating the setting of the switches to the 23dB position).

The two switches attached to this stage are:

  • Preamp gives an additional 15 dB of gain in the RF input. Probably only needed on 20 meters.
  • Attenuator is self-evident. It’s purpose is to allow the radio to function in a very intense RF environment. Rarely needed.

Temporarily enable IQ Output photo
tsw1_2shunt wire (cut-off lead)Cutoff 
temporary connection - see diagram

Inspect Completed Board

Inspect Completed Board photo

RX Mixers/OpAmps Testing

NE5532 Pin Voltages

Test Setup

Apply 12 - 13.8 Vdc power to the board and measure, in turn, the pin voltages on each NE5532

Each should yield the following pin voltages:

  • Pins 1, 2, 3, 5, 6, and 7: +6Vdc
  • Pin 4: (gnd) 0 Vdc
  • Pin 8: = Vin = 12Vdc

The 6 Vdc reading above is nominal and will actually depend upon your power source. Some will use 13.8 Vdc; some may have just 12V dc (as in the typical gel cell). The important thing is that the voltages (nominally 6V dc) must be the same on each of the indicated pins.

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Each NE5532: pins 1, 2, 3, 5, 6, and 7V dc+6tdb_______
Each NE5532: pin 4V dc0tdb_______
Each NE5532: pin 8V dc+12tdb_______

Image Adjustment

Test Setup

At this stage it is absolutely essential to ensure that both I and Q channels are working properly and image rejection is adjusted (-40dB or better).

"Images" are a common artifact of the Direct Conversion receiver which is the basis for this SDR. They are encountered in the display when:

  • the phasing of the I and Q signals is not just right; OR
  • when only one of the two signals is present, either because of a bad connection or a mono soundcard input).

Images are pretty obvious on the spectrum display of the SDR, looking like mirror inage signals on either side of the center frequency. One of these is the desired signal; the other is an image that should be rejected. For example, if the center frequency is 10.125 MHz and your desired signal is 10.135 MHz (10 kHz above the center "zero"), then there may be a mirror image of that signal at 10.115 kHz (10 kHz below the center "zero"). Adjusting for image rejection causes these unwanted images to disappear down into the noise level.

To begin, Download and setup Winrad to serve as a SDR receiving program (you may prefer, instead, to use the GSDR program - Hans, LA2MOA's setup instructions are here.

Image adjustment will require a running SDR program tuned to one of the center frequencies and a signal source that is slightly above or below that center frequency. For this example, we will use the 30m center frequency of 10.125 MHz and a signal source of 10.115 MHz (10 kHz below the center). Preston, WJ2V, has found that if the test signal was too large, it was picked up in the unshielded G3020, and it would not suppress! Use a small signal to do these tests. When Preston got the signal down to proper levels for receive testing, the supression worked like magic.

The adjustment procedure below is extracted from Preston's excellent FAQ

Adjustment Procedure

The G3020 has four pots that must be adjusted to balance the audio lines for proper operation.

Adjustment trim pots

Set Up Sound Card
Start up your Genesis software, and connect your sound card to the G3020:
  • Connect line In of PC to the "To PC Soundcard" jack on the G3020 board
  • Do not connect the soundcard's line out to the G3020 yet
Signal Source

You will need a source of low signal in either the 30 or 20 meter bands. You must keep the signal level quite low or the Genesis will be overloaded, and you won’t see the nulls you are looking for. Builders have reported inability to find a null when the signal strength is too high. For example, you may have trouble with a standard 100 watt rig connected to a dummy load—the escaped signal may still be pretty high. If necessary, move your G3020 away from the signal source. Of course, a signal generator would be ideal, but it isn’t necessary.

Adjust Carrier Trim Pots

Now, look at the above graphic. If you printed this out in advance, it will be easier than trying to look at your computer while you work. Note there are two 5k (Y503) trim pots labeled "TX Carrier Suppression". These pots must be pre-adjusted. Locate them on your board, so you are oriented.

  • Power the board so you can make some voltage measurements with respect to ground (WRTG)
  • You will find that the trimmer pot at the top of your board has a 1K resistor right below it. The right side of this resistor (red dot) is connected to the wiper of this trimmer. Adjust that trim pot to exactly 2.50 volts (WRTG) with power on the board.
  • Now go to the second Carrier Null trimmer. The wiper of this trimmer is connected to the left side (red dot) of the 1K resistor just to the right of the trimmer. Adjust this second trimmer for 2.50 volt (WRTG) as well.
  • Note that although these two pots are ultimately used to null out the carrier, they must be very close to equal, or you will not be able to suppress the image in the receiver.
RX Image Adjustment

Next trimmer is the 500 ohm (Y502) receiver image adjustment trimmer, for which you will need a small signal. Staying with our 30m example, your G3020 uses a local oscillator at 10.125 MHz. If you generate a signal at 10.115, which is 10K below the LO center, the image will be on 10.135, 10K above. If you generate a signal at 10.135, the image is on the other side, i.e. 10.115.

So, now look at your SDR software's spectrum display onm the PC and note the two signals, the correct signal, and the wrong side image.

Turn power OFF. Referring to the trimmer adjustment graphic find the receive image suppression pot. With power off, pre-set it so that the resistance across the 330 ohm resistor (to the right of the pot) is 150 ohms, as shown in the above graphic

Once your image suppression pot is pre-adjusted, you can carefully turn it until the image starts to drop. It should show at least 30dB, more likely 40dB of null. If the wrong signal is dropping, go in to the your SDR software's setup, and reverse the receive I/Q lines to correct this problem. Now, the image should be suppressed.

Turn the power on, fire up your SDR software for the G3020 and start up your signal source. Then, adjust the image reject trimmer while observing the "image" signal on the spectrum display. Refer to the procedure outlined for the G40 kit's image adjustment to see what the image rejection process looks like. You will be using different input frequency with your G3020, but the rest of the instructions apply. You can also get a very good idea of how this works by viewing the You Tube video by Josh, Vk2FJDX (fast forward to minute 4:40).

Also, you might find that the Genesis software is easier to use than others.

Image Adjustment

Watch One Work

Test Setup

Here is a Youtube video of Brian KJ4OOS' Genesis G3020 receiving SSB signals at the end of this Phase: