Ensemble RXTX - RX Opamps and Output
Band: 30, 20, 17m

Introduction

Summary Build Notes

1. Install Bottomside Caps and IC
2. Install Topside Resistors and Caps
3. Install Jack and I/Q Shunts
4. Test the Stage

Stage Schematic

Board Layouts

Board Top

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Board Bottom

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RX Opamps and Output Bill of Materials
(30, 20, 17m band option)

(details for installation of each component are provided in the step instructions, further down the page)

Check	Type	Category	Component	Count	Marking	Image
☐	Capacitor	Ceramic	220 pF 5%	2	221
☐	Capacitor	SMT 1206	0.1 uF	3	(smt) black stripe
☐	Capacitor	Ceramic	4.7 uF 10% 16V X7R RAD	1	475
☐	Connector	Jack-RA	3.5mm stereo jack - PCB mount (rt-angle)	1
☐	IC	SOIC-8	LT6231 dual op-amp	1	LT6231 ESD!!!
☐	Resistor	1/6W	100 1/6W 5%	2	br-blk-br-gld
☐	Resistor	1/4W	4.99 k 1/4W 1%	2	y-w-w-br-br
☐	wire	Cutoff	shunt wire (cut-off lead)	2

Detailed Build Steps

Step_Install Bottomside Caps and IC

Check	Designation	Component (top/bottom)	Orientation	Marking	Image	Band	Notes
☐	C73	0.1 uF ((bottom))		(smt) black stripe		any
☐	C74	0.1 uF ((bottom))		(smt) black stripe		any
☐	C75	0.1 uF ((bottom))		(smt) black stripe		any
☐	U11	LT6231 dual op-amp (top)	(or LT6221)	LT6231 ESD!!!		any	Some kits may substitute 6221

Step_Install Topside Resistors and Caps

Note: the two 100 ohm resistors (R57 and R60) in series with the ouputs are there because the op-amps don't like to drive purely capacitive loads - it degrades the phase margins.

Note also that these are the smaller 1/6Watt resistors; not 1/4 watt variety.

Check	Designation	Component (top/bottom)	Orientation	Marking	Image	Band	Notes
☐	C43	4.7 uF 10% 16V X7R RAD (top)		475		any
☐	C44	220 pF 5% (top)		221		any
☐	C45	220 pF 5% (top)		221		any
☐	R56	4.99 k 1/4W 1% (top)	N-S	y-w-w-br-br		any
☐	R57	100 1/6W 5% (top)	E-W	br-blk-br-gld		any
☐	R59	4.99 k 1/4W 1% (top)	N-S	y-w-w-br-br		any
☐	R60	100 1/6W 5% (top)	E-W	br-blk-br-gld		any

Step_Install Jack and I/Q Shunts

In both the RX op amp and TX QSE sections, the jumper links on the I/Q input and outputs should ideally be fitted crossed over. That provides compatibility with all versions of PowerSDR. For Rocky and Winrad it is necessary to use the 'swap IQ' functions in their set up. If you jumper them straight across fine for Rocky and Winrad, but you will not be able to use any version of PSDR. (Author wired for Rocky)

Some builders (author included) have encountered issues in this setup and discovered that the only the TX leads should be crossed, leaving the RX leads uncrossed. Be prepared to change these should you encounter this issue.

Also, do not be confused with the terminology here:  "Line-In (as silkscreened on the PCB" signifies a connection point for connecting to the Sound card's "Line-in jack".  Remember: for the RXTX, "Line-In" goes to "Line-In" and "Line-Out" (see TX OpAmps) goes to "Line-Out"

Check	Designation	Component (top/bottom)	Orientation	Marking	Image	Band	Notes
☐	J4	3.5mm stereo jack - PCB mount (rt-angle) (top)				any
☐	JP2a	shunt wire (cut-off lead) (top)				any
☐	JP2b	shunt wire (cut-off lead) (top)				any

Completed Photos

View of Completed Topside

View of Completed Underside

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Test the RX Opamps and Output Stage

RX Opamps and Output - Current Draw

Measure the current draw of the board with both USB and regular power applied.

Test Steps (if any)

RX Opamps and Output - Pin Voltage Tests

Test pin voltages WRT regular ground, as per graphic

As usual, if you are having any problems with this stage, be sure to measure the pin voltages two ways: first, measure each pin at the pin itself (on the IC). Then take a second measurement at the pin pad (on the board). If those two measurements do NOT agree, you very likely have a soldering issue.

Test Steps (if any)

RX Opamps and Output - OpAmp Function Test

Test Setup

In this test, you will test the DC gain of each of the op-amps by connecting a 10k bridging resistor R_b from each op-amp inverting input to circuit regular ground. Introducing the "bridging" resistor R_b will result in a test current equal to 2.5 / R_t, which will be balanced by the current fed back from each op-amp's output through each feedback resistor, R_f (i.e., R56 or R59). Each op-amp output will increase in voltage by 2.5 * R_f/ R_b from the nominal DC level of 2.5 volts.

(If you have not yet built the TX Op Amp stage, the10k bridging resistor (R_b) will be available for your use)

Test the First OpAmp

1. Power up the circuit and measure the voltage at pin 1 of the op-amp (hairpin of R59). It should be ~2.5 Vdc
2. Power off and use clip leads to connect R_b between the hairpin of R58 and circuit ground (the left lead of C43 is a convenient ground). This provides an input resistance(R_i) of10 kΩ to the op-amp.
3. Power up and measure the output voltage (WRT regular ground) at the hairpin of the feedback resistor R59. You should get: ~3.75 Vdc at R59 hairpin.
4. Remove R_b and the output voltage at R59 should go back to ~2.5 Vdc.

Test the Second OpAmp

1. Power up the circuit and measure the voltage at pin 7 of the op-amp (hairpin of R56). It should be ~2.5 Vdc
2. Power off and use clip leads to connect R_b between the hairpin of R55 and circuit ground (the left lead of C43 is a convenient ground). This provides an input resistance(R_i) of 10 kΩ to the op-amp.
3. Power up and measure the output voltage (WRT regular ground) at the hairpin of the feedback resistor R56. You should get: ~3.75 Vdc at R56 hairpin.
4. Remove R_b and the output voltage at R56 should go back to ~2.5 Vdc.

The diagram below shows the test points. The yellow dots show the R_b connection points for each "side" of the opamps. The dots marked "A" and "B" show the measurement points for the output voltages for Each "side" of the OpAmps.

An Excel spreadsheet with a calculator for this test is available for you to plug in your bridging resistor ohms (R_t) and your pin 1 or pin 7 normal voltages (E_bias) and predict the expected voltage when bridged (E_out).

You can use the simple calculator below for this test. All you need to do is plug in your bridging resistor ohms (R_t) and your pin 1 or pin 7 normal voltages (E_bias) and predict the expected voltage when bridged (E_out).

Test Steps (if any)

RX Opamps and Output - RX Test

Prepare an SDR program for RX (author recommends Rocky for the Windows XP crowd; WinradHD for other Windows OS). This usually involves downloading and installing the program; selecting the desired soundcard for the (STEREO) input of the I and Q signals from the board; and connecting the board to the soundcard with a stereo cable with 1/8" stereo plugs on either end.  

The stereo cable should be connected between the PC sound card's "Line-In" input and the Ensemble RXTX's (output) jack (silkscreen-labeled "Line-In" on the board).

Once the SDR program is ready, connect the USB cable from your PC to the board, connect the 12V power to the board, and connect a 50 ohm antenna to J6.

Start the SDR program and adjust the LO frequency to the desired center frequency. You should see signals in the displayed spectrum. If there is a contest going on at test time, you will be even more impressed with the RX.!

Note: for either SDR program to work with the Ensemble RXTX, you MUST have installed the driver (libusb) for the Microcontroller's USB functionality. For WinradHD, you will also need to have downloaded and saved in the same directory as WinradHD, the EXTIO_Si570.dll file.

If you are seeing perfect mirror inages of the signals either side of the center frequency, you should review the information in the Image Rejection Hints page.

Ground Loops and Humps

If you experience a large "hump" at the center frequency or several spurii on the spectrum, you may be the victim of ground loops.

Alan G4ZFQ has an excellent article on the effects of ground loops on the SDR and how to minimize/avoid them. (OK, Alan, so it's "minimise"!)

Test Steps (if any)