RX OpAmps Introduction


In this phase, the down-converted audio frequency signals (in quadrature) from the mixersare combined (0° + 180°, 90° + 270 °) and then summed and passed to the PC, either directly following the summing operatikon or passed through an amplification stage for a 23 db boost (as determined by S1), as I and Q outputs of the G40.

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.) The relays, designated in the schematic below as K1 and K2, act to switch the I/Q signal pairs from the default RX to the TX signals when activated by the voltage at Vtx. Vtx is present when the PTT is activated via the RX/TX Control circuitry. In the final wiring diagram, this Vtx is provided by a wire whose end-points are identified by the letter "G". In the final wiring diagram.

RX OpAmps Schematic

RX OpAmpsschematic

RX 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  RX OpAmps
1210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W  RX OpAmps
4100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W  RX OpAmps
1150 1/4W 1%brn-grn-blk-blk-brn brn-grn-blk-blk-brn1/4W  RX OpAmps
133 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W  RX OpAmps
1330 1/4W 1%ora-ora-blk-blk-brn ora-ora-blk-blk-brn1/4W  RX OpAmps
23k3 1/4W 1% (3.3K)ora-ora-blk-brn-brn ora-ora-blk-brn-brn1/4W  RX OpAmps
247K 1/4W 1%yel-vio-blk-red-brn yel-vio-blk-red-brn1/4W  RX OpAmps
2JRC27F/012 relayJRC27F/01212V relay  RX OpAmps
110 nF (.01uF)103 103Ceramic  RX OpAmps
15100 nF104 104ceramic  RX OpAmps
1100 nF USM104J 104JCeramic  Likely is C61 on schematicRX OpAmps
1100 nF USM104J 104JCeramic  likely is C62 on schematicRX OpAmps
4NE5532 Dual OpAmp DIP-8 RX OpAmps
1DPDT SwitchDPDT Connects as S1A and S1B RX OpAmps
110 uF/25Vdc Electrolytic RX OpAmps
12100 uF/25Vdc Electrolytic  RX OpAmps
2220 uF/16Vdc Electrolytic  RX OpAmps
13.5mm stereo jack - PCB mountjack  RX OpAmps
1BNC Connector Male - PCB mountjack  RX OpAmps
1misc hookup wiremisc  RX OpAmps
1shunt wire (cut-off lead)misc  RX OpAmps
410 uF (NP) Non-Polarized  RX OpAmps
1500 (Y501)Y501trimpot  RX OpAmps
25K (Y502)Y502 Y502trimpot  Needs to be initialized. Before installing this trimmer, adjust the resistance between the middle pin and either of the other pins to 2.5 K Ohm.RX OpAmps

RX OpAmps Summary Build Notes

RX OpAmps Detailed Build Notes

Top of the Board

RX OpAmps Top View

Install the Capacitors

A number of capacitors, electrolytc and non-electrolytic are installed for the RX Opamp, per the graphic

Install the Capacitors photo
E6610 nF (.01uF)103 103Ceramic 
E9010 uF/25Vdc Electrolytic 
C3100 nF104 104ceramic 
E120100 nF104 104ceramic 
E155100 nF104 104ceramic 
E170100 nF104 104ceramic 
E172100 nF104 104ceramic 
E248100 nF104 104ceramic 
E258100 nF104 104ceramic 
E26100 nF104 104ceramic 
E268100 nF104 104ceramic 
E343100 nF104 104ceramic 
E43100 nF104 104ceramic 
E72100 nF104 104ceramic 
E74100 nF104 104ceramic 
E76100 nF104 104ceramic 
E80100 nF104 104ceramic 
E11410 uF (NP) Non-Polarized 
E15710 uF (NP) Non-Polarized 
E16510 uF (NP) Non-Polarized 
E7710 uF (NP) Non-Polarized 
E129100 uF/25Vdc Electrolytic 
E132100 uF/25Vdc Electrolytic 
E151100 uF/25Vdc Electrolytic 
E156100 uF/25Vdc Electrolytic 
E207100 uF/25Vdc Electrolytic 
E255100 uF/25Vdc Electrolytic 
E259100 uF/25Vdc Electrolytic 
E261100 uF/25Vdc Electrolytic 
E275100 uF/25Vdc Electrolytic 
E295100 uF/25Vdc Electrolytic 
E71100 uF/25Vdc Electrolytic 
E85100 uF/25Vdc Electrolytic 
E127100 nF USM104J 104JCeramic 
Likely is C61 on schematic
E344100 nF USM104J 104JCeramic 
likely is C62 on schematic
E133220 uF/16Vdc Electrolytic 
E265220 uF/16Vdc Electrolytic 

Install VK6IC Fix (optional)

For Boards Serial Numbered from 1 to 100 (" VK6IC Fix")

Boards numbered from 1 to 100 were shipped without on or more of the modifications in the pdf file describing the fix by VK6IC. This fix calls for cutting one trace and soldering a small jumper (both in the underside of the board, in the vicinity of pins 14 and 13 of the top 74HC4066 Quad Bilteral Switch IC.

The original board design had pin 13 connected to a bypass capacitor. The modification removes that connection and makes a connection between pin 14 and the bypass capacitor. Some boards in the higher numbers of the 1-100 range have already had the trace cut but still need the jumper wire soldered, as in the case of the author's board.

Install VK6IC Fix (optional) photo
fixshunt wire (cut-off lead)misc 

Install Resistors, etc.

Similarly, a large number of resistors, along with three pots and 2 relays, are installed for the rx opamps, per the following diagram:

Install Resistors, etc. photo
E3033 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
E106100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
E158100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
E171100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
E7100 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
E109150 1/4W 1%brn-grn-blk-blk-brn brn-grn-blk-blk-brn1/4W 
E160330 1/4W 1%ora-ora-blk-blk-brn ora-ora-blk-blk-brn1/4W 
E1081 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E1131 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E1591 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E1631 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E251 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E2601 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E2641 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E611 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E631 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E671 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
E1353k3 1/4W 1% (3.3K)ora-ora-blk-brn-brn ora-ora-blk-brn-brn1/4W 
E2573k3 1/4W 1% (3.3K)ora-ora-blk-brn-brn ora-ora-blk-brn-brn1/4W 
E11010 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E11210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E11510 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E11610 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E11710 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E11810 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E16110 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E16210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E16610 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E16710 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E16810 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E16910 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W 
E13647K 1/4W 1%yel-vio-blk-red-brn yel-vio-blk-red-brn1/4W 
E25647K 1/4W 1%yel-vio-blk-red-brn yel-vio-blk-red-brn1/4W 
E10JRC27F/012 relayJRC27F/01212V relay 
E9JRC27F/012 relayJRC27F/01212V relay 
E296500 (Y501)Y501trimpot 
E175K (Y502)Y502 Y502trimpot 
Needs to be initialized. Before installing this trimmer, adjust the resistance between the middle pin and either of the other pins to 2.5 K Ohm.
E185K (Y502)Y502 Y502trimpot 
Needs to be initialized. Before installing this trimmer, adjust the resistance between the middle pin and either of the other pins to 2.5 K Ohm.

Install Ics NE5532

The four NE5532s that provide the RX OpAmp chain are installed as the center two chains of an array of four chains of NE5532s. The builder needs to take care to install in the RX opamps' holes, not the TX opamps's holes. The builder must also pay close attention to the orientation of the opamps. They are installed such that their printing is upside down, with the marking dot oriented "top right".

Some have asked whether they could/should use sockets. The answer has been "It is OK to use sockets for these ICs, but the sockets must be of the highest quality" (a large cost issue).

If the builder is NOT using sockets, follow the maxim "Measure twice, cut once", since these brutes are hard to get out once soldered in.

Install Ics NE5532 photo
E153NE5532 Dual OpAmp DIP-8
Take ESD precautions
E154NE5532 Dual OpAmp DIP-8
Take ESD precautions
E69NE5532 Dual OpAmp DIP-8
Take ESD precautions
E70NE5532 Dual OpAmp DIP-8
Take ESD precautions

Adjust Trimmer

With the power OFF, adjust trimpot Y501 until the resistance across the 300 ohm resistor is 150 ohms.

Adjust Trimmer photo

Install Antenna Connector

Install the bnc jack on the left-hand edge of the board

Install Antenna Connector photo
E137BNC Connector Male - PCB mountjack 

Install RX I/Q Jack And Switch

  • RX I/Q Audio Jack
    Mount the stereo jack on the right-hand side of the board per the graphic.
  • Additional Op-Amp Switches, S1a and S1b
    Then, connect the DPDT Switch S1 to the two 3-pin points on the right-hand side of the board, such that each of the wires for each of the (A and B) of pins on S1 line up: "1" on the switch with "1" on the board, "2" with "2" , etc.
    These switches allow you to add another op-amp stage to the RX chain, boostongf the gain from 23dB to 46dB.

Install RX I/Q Jack And Switch photo
E141DPDT SwitchDPDT 
Connects as S1A and S1B
E1373.5mm stereo jack - PCB mountjack 

Install Temporary Antenna Jumper

"Tack" solder a wire to connect the antenna "hot" pin on the BNC connector to the Mixer Stage's Input point on the board

The wire connects the antenna "hot" terminal in the BNC connector to the point where a not-yet-installed relay will switch the antenna in/out of the TX /RX path(s). Soldering to the BNC is simple; soldering to the relay pin should be done carefully so as to "tack" the wire to the hole's surrounding pad, yet NOT fill the hole up with solder, thus, making the subsequent installation of the relay considerably easier.

Install Temporary Antenna Jumper photo
wire5misc hookup wiremisc 

RX OpAmps Testing

Current Draw

Test Setup

Check RX current consumption - it should low, around 120 mA.

Much higher current readng will indicate leakage to ground

Very low current usually means no connection (e.g., a blown diode somewhere in the circuit).

Voltage Tests - 6V

Test Setup

Power up the board

Measure the voltages on each of the NE5532 OpAmps. Each OpAmp's voltages must be the same for each corresponding pin (see the table below)

When all OpAmps are working correctly, the +voltage is determined by your input voltage. Typically the voltage at pins 1-3 and 5-7 is around +6 to 6.5V with +13.8Vin and around +5.5V with +12Vin.

The exact voltage on the NE5532 (pins 1-3 and 5-7) is not critical at all. Any value around +6V is just fine. However what really matter is that the voltage on all pins of all four NE552s is identical or at least symmetrical.

This information is important for us for two practical reasons:

  • Firstly, it tells us whether all the resistors around the OpAmps have been installed correctly.
  • Secondly, it tells us if all the NE OpAmpss are working properly- otherwise proper I / Q shift adjustment may not be possible.
When all OPamps are working correctly, +voltage is determined by your input voltage. Typically around +6 to 6.5V with +13.8Vin and around +5.5V with +12Vin. Voltage Tests - 6V

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
All NE5532: Pins 1, 2, 3, 5, 6, 7V dc6.05.9 (Vcc=12.6)_______
All NE5532: Pin 4V dc00_______
All NE5532: Pin 8V dc1211.2 (Vcc=12.6)_______

2.5V dc Tests

Test Setup

With power on, measure the voltage at the "+2.5" testpoints (white circles). The voltage should be approximately 50% of the 5 V rail voltage

2.5V dc Tests

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Point 2Vdc2.52.45_______
Point 1Vdc2.52.49_______

Mixer Pin Voltages

Test Setup

With power on, measure the voltage at the designated pins (see table below) of each mixer IC (74HC4066).

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Pin 5V dc2.52.49_______
Pin 6V dc2.52.49_______
Pin 12V dc2.52.49_______
Pin 13V dc2.52.49_______
Pin 14V dc55.02_______

RX Testing - Image Adjustment

Test Setup

Connect an antenna to the BNC antenna jack on the G40

Connect the G40 to your PC using plug cables between the RX I/Q jack and the PC's Stereo Line-In input (or MIC input, if the PC has a stereo MIC input).

Start your SDR program of choice and set the center frequency at 7.047 MHz (you may want to use the GSDR program - Hans LA2MOA's setup instructions are here.

Power on the G40.

You should see some 40m signals if your RX is working. See the video of :Bojan S51QA's test at this point.

Image Rejection Adjustment

It is crucial that after the build phase 5 image rejection is adjusted properly.

"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 7.047 MHz and your desired signal is 7.057 MHz (10 kHz above the center "zero"), then there may be a mirror image of that signal at 7.037 MHz (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

Image adjustment will require a running SDR program tuned to 7.047

Images are eliminated by properly adjusting the hardware, using the techniques described on the G40 website. Follow these techniques to correctly adjust your receiver.

(See also the excellent Video by Josh VK2FJDX (fast forward to minute 4:40). If you don't understand that, you need help!)