Ensemble RX II (HF/LF) Operational Amplifiers
Band: HF

Stage Schematic

(Red dots represent the "hairpin" (or left-hand or topmost) lead of the component)

Board Layouts

Board Top

Board Bottom

Summary Build Notes

1. Install Bottomside Parts
2. Install Topside Parts
3. Test the Stage

Operational Amplifiers Bill of Materials
(HF 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	390 pF 5%	2	391
☐	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	120 1/6W 5%	2	brn-red-brn-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

Install Bottomside Parts

Check	Designation	Component (top/bottom)	Orientation	Marking	Image	Band	Notes
☐	U11	LT6231 dual op-amp ((bottom))	(or LT6221)	LT6231 ESD!!!		any
☐	C45	0.1 uF ((bottom))	white pads	(smt) black stripe		any
☐	C46	0.1 uF ((bottom))	white pads	(smt) black stripe		any
☐	C47	0.1 uF ((bottom))	white pads	(smt) black stripe		any

Install Topside Parts

Careful - there are 1/6W and 1/4W resistors in this step.

When installing the jumpers to the line-in jack, the builder is advised to install them crosswise (i.e., in the "X" pattern), rather than straight (in the parallel pattern), using insulated hookup wire to avoid shorts. This will make the receiver adaptable to all currently published SDR software. While the outputs would appear reversed to Rocky and Winrad, those two programs have a programmatic "switch I and Q lines" setting that can compensate. The PSDR versions do not have such a setting.

Check	Designation	Component (top/bottom)	Orientation	Marking	Image	Band	Notes
☐	C22	4.7 uF 10% 16V X7R RAD (top)	horiz	475		any
☐	C24	390 pF 5% (top)	vert	391		any
☐	C23	390 pF 5% (top)	vert	391		any
☐	R32	4.99 k 1/4W 1% (top)	N-S	y-w-w-br-br		any	Take care not to use the 49.9 ohm resistors.
☐	R33	4.99 k 1/4W 1% (top)	N-S	y-w-w-br-br		any	Take care not to use the 49.9 ohm resistors.
☐	R34	120 1/6W 5% (top)	E-W	brn-red-brn-gld		any
☐	R35	120 1/6W 5% (top)	E-W	brn-red-brn-gld		any
☐	JP1A	shunt wire (cut-off lead) (top)				any
☐	JP1B	shunt wire (cut-off lead) (top)				any
☐	J02	3.5mm stereo jack - PCB mount (rt-angle) (top)				any

Completed Photos

View of Completed Topside

View of Completed Underside

Test the Operational Amplifiers Stage

Operational Amplifiers - 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.

If you are measuring zero volts where expected value is 2.5V,  check the voltage at R28 hairpin again (this was checked in the Auto Bandpass Filter stage and should have been ~2.5 Vdc in that check.  If it is now zero, check for shorts.  Also check for the correct values in R32 and R33.  If these were accidentally installed as 49.9 ohms vs 4.99K ohms, the voltage at R28 hairpin could approximate zero .

Test Steps (if any)

Operational Amplifiers - Functional Test

Test Setup

Test the First OpAmp

1. Power up the circuit and measure the voltage at pin 1 of the op-amp (hairpin of R33). It should be ~2.5 Vdc
2. Power off and use clip leads to connect R_b between the hairpin of R31 and circuit 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 R33. You should get ~3.75 Vdc at R33 hairpin.
4. Remove R_b and the output voltage at R33 should go back to ~2.5 Vdc.

Test the Second OpAmp

1. Power up the circuit and measure the voltage at pin 1 of the op-amp (hairpin of R32). It should be ~2.5 Vdc
2. Power off and use clip leads to connect R_b between the hairpin of R30 and circuit ground. This provides an input resistance(R_i) 10 kΩ, to the op-amp.
3. Power up and measure the output voltage (WRT regular ground) at the hairpin of the feedback resistor R32. You should get: ~3.75 Vdc at R32 hairpin.
4. Remove R_b and the output voltage at R32 should go back to ~2.5 Vdc.

The diagram above show 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).

If your results are not as nominally expected, double check the values of R32 and R33 (both 4.99k, NOT 49.9). See this message thread on the Yahoo Softrock40 reflector.

Test Steps (if any)

Operational Amplifiers - 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..

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 J4.

Connect the "Line-In" jack on the pc board to the "Line-In" input jack on the pc or sound card, using a length of male-to-male stereo audio cable, with 18/" stereo plugs

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 RX, 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.

If you installed the I/Q jumpers as suggested, you will need (in Rocky and/or Winrad) to switch the I and Q inputs to get signals properlay arrayed on the correct "side" of the center frequency.

Note that there is an optional grounding via for the return side of the antenna connection. You do NOT want to connect this to the SDR ground per the advice in the webpage referenced below:).

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"!)

Another great resource from Alan is his page on "SDR with SoundCard Basic Faults" (which graphically shows what poor grounding can do in a sound-card-based SDR application).

Test Steps (if any)