Part Number: LMX2592
Other Parts Discussed in Thread: , LMX2595, LMX2594, LMX2572
Hello TI team,
I am planning to use the LMX2592 as a replacement for the Analog Devices HMC832 (PLL + integrated VCO) in an existing radio system. I would like to confirm the recommended usage for the OSC input, RF outputs, and the handling of pin 30.
1. Application background
- Existing device: HMC832 (PLL + integrated VCO)
- Replacement candidate: LMX2592
- One equipment uses 5 × LMX2592 devices.
- Each device output frequency is as follows:
1) 353.7 MHz (fixed)
2) 340.0 MHz (fixed)
3) 1795–1995 MHz, or 1845–1945 MHz
4) 1820–2020 MHz, or 1870–1970 MHz
5) 1262.5 MHz (fixed)
I am currently designing the replacement PCB and need to decide how to connect the OSC input pins and RF outputs, and how to handle pin 30.
2. OSC input (reference clock) – single-ended use
In my design, I would like to use a single-ended reference clock to drive the OSC input.
However, in the LMX2592 datasheet (Rev.C) and in the LMX2592EVM schematic (SNAU195), the OSC inputs (OSCinP/OSCinM) are always shown as differential inputs. I could not find a clear description of single-ended operation for OSCin.
Q1. For single-ended operation, can I drive only one of the OSC input pins?
- For example: drive OSCinP with AC coupling, and leave OSCinM unused.
- Or, alternatively: drive OSCinM with AC coupling, and leave OSCinP unused.
- Is there any restriction that I must use P side (OSCinP) only, or can I use either P or M?
Q2. If I use only one OSC input pin (e.g., OSCinP) for a single-ended signal, what is the recommended treatment for the other pin (OSCinM)?
- Is it acceptable to leave OSCinM open (no connection)?
- Or should OSCinM be AC-coupled to GND (for example, via a capacitor to create an AC ground)?
- Is there any official recommendation or application note for using LMX2592 with a single-ended reference clock?
If there is a recommended example circuit (even from a similar device family like LMX2594/LMX2595/LMX2572), I would appreciate a reference.
3. RF outputs (RFoutA / RFoutB) and single-ended use
In my application, for each LMX2592 device, I will use only one RF output path (either RFoutA or RFoutB). I plan to use this RF output as a single-ended signal.
Q3. Is there any performance difference between RFoutA and RFoutB?
For example, in terms of:
- usable frequency range,
- output power range,
- phase noise,
- spurious performance, etc.
Or can I treat RFoutA and RFoutB as functionally equivalent, and simply choose one of them based on PCB layout convenience?
Q4. For the RF output path that I do not use (for example, RFoutB), is the following procedure correct?
- Disable RFoutB by register setting (power-down / output disable), and
- Leave the RFoutB pins (RFoutBP, RFoutBM) unconnected on the PCB.
Is this the recommended way to treat the unused RF output channel?
4. Single-ended RF output from differential pins
When I use RFoutA (or RFoutB), I would like to use single-ended output from the differential pair.
Q5. For single-ended RF output, can I use either RFoutAP or RFoutAM (or RFoutBP / RFoutBM)?
- For example, is it acceptable to use RFoutAP → AC-coupling capacitor → 50 ohm trace, and leave RFoutAM unused?
- Or can I use RFoutAM instead in the same way?
If there is a recommended choice (for example, “use P side for single-ended output”), please let me know.
Q6. For the other side of the differential pair (e.g., RFoutAM when I use RFoutAP as single-ended output), what is the recommended treatment?
- Is it acceptable to leave the unused RF pin open (no connection)?
- Or should it be terminated in some way (for example, AC-grounded or 50 ohm termination)?
If there is an official recommendation or application note for using LMX2592 RF outputs in single-ended mode, please let me know.
5. Pin 30 handling (NC vs 680 ohm to GND on EVM)
Regarding pin 30, I see a discrepancy between the LMX2592EVM schematic and the latest datasheet.
- In the LMX2592EVM schematic (SNAU195), pin 30 is connected to GND through 680 ohm.
- In the LMX2592 datasheet Rev.C, pin 30 is marked as NC (No Connect).
Q7. For new designs, what is the correct and recommended treatment for pin 30?
- Is it safe and recommended to leave pin 30 unconnected on the PCB (no trace, no pull-down resistor), following the datasheet Rev.C (NC)?
- Or should we follow the EVM schematic (680 ohm to GND)?
In other words, for a new design using the latest LMX2592 silicon and datasheet Rev.C, is “leave pin 30 floating (no connection)” the correct implementation?
6. References
- LMX2592 datasheet (Rev.C)
- LMX2592EVM User’s Guide: SNAU195
If there are any updated documents or application notes that describe:
- single-ended OSC input usage, and/or
- single-ended RF output usage
for LMX2592, I would be very grateful if you could point me to them.
Thank you very much in advance for your support.
Best regards,
Ryusuke
Ryusuke
